The Senate voted 78-22 to authorize President Barack Obama’s plan.
Obama later thanked lawmakers for approving his request.
“As I said last week, I believe that we’re strongest as a nation when the president and Congress work together, and I want to thank members of Congress for the speed and seriousness with which they approached this important issue,” he said.
Earlier, Defense Secretary Chuck Hagel told a House panel that top U.S. military leadership approved a plan to strike ISIL targets inside Syria.
Hagel said Obama was briefed on those plans, approved by Hagel and the chairman of the Joint Chiefs of Staff, during his visit to U.S. Central Command on Wednesday. Obama has not signed off on those plans yet, CNN has learned.
The so-called Islamic State continues to spread its radical Islamist rule in Syria and Iraq and on Thursday captured 16 predominantly Kurdish villages in northern Syria over the past 24 hours, a Syrian opposition group said. ISIL used artillery and tanks against the villages along the Syria-Turkey border, the Syrian Observatory for Human Rights said.
And the group published a video in which British hostage John Cantlie criticizes the American and British governments for their failure to negotiate for the hostages as other governments have done. He goes on to make a number of other propaganda points and promises a series of similar video presentations.
Since Cantlie is delivering ISIL propaganda and makes clear in the video he is speaking under duress, CNN will not show the video.
Meanwhile, U.S. lawmakers are debating the best way to stop ISIS, with Secretary of State John Kerry testifying before the House Foreign Affairs Committee. He said that while Syria had removed most of its chemical weaponry, President Bashar al-Assad continues to use chlorine weapons and is "in violation" of a treaty against the use of such weapons.
On Wednesday, the House approved Obama's plan to arm and train Syrian rebels. The House then tacked that onto a government spending bill that would allow leaders to continue funding the U.S. government and sent it to the Senate. On Thursday, the Senate will be voting on that whole package, fueling criticism that lawmakers are avoiding a separate vote on arming the Syrian rebels.
Sen. Susan Collins, a Republican from Maine, told CNN's "New Day" that she had serious concerns.
"I'm going to vote for the continuing resolution because I don't want government to shut down," she said. "Right now, the Senate isn't even scheduled to have a separate vote on the Syrian resolution, and that's just plain wrong."
U.S. military on deck
The U.S. military has everything it needs to strike ISIL inside Syria and is waiting on the President's authorization to move forward, U.S. military officials told CNN on Thursday.
For weeks, intelligence and military targeting specialists have been working around the clock on a list of targets. The list is expected to be shown to Obama one more time. An analysis of the risks of bombing inside Syria will be included, as well as an assessment of how the destruction of the targets could degrade ISIS, officials said.
It is most likely that the President will get a broad description of the list, with some analysis of what would be accomplished. Presidents generally do not review each and every target before a strike. General guidance is given, and then the military selects the time, date and place after the President makes the political decision to proceed.
Obama is "actively" reviewing options and has "offered guidance" to the Department of Defense about the target sets that he's reviewed, White House spokesman Josh Earnest said last week. But Earnest said that Obama is not signing off on each strike.
Meanwhile, the U.S. military said it conducted more airstrikes in Iraq on Wednesday and Thursday. One strike near an ISIS training camp southeast of Mosul destroyed an ISIS armed vehicle, two occupied buildings and a large ground unit. Another strike southeast of Baghdad damaged an ISIS ammunition stockpile, according to a news release.
Some Democrats leery of arming rebels
Even though some senators expect Obama's plan to pass in the Democrat-controlled Senate, the President could see tough challenges from his own party.
"I think it's very hard to sort out the moderate rebels from the extremists, and I have a real worry that once we send these rebels back into the battle space, there is very little we can do to prevent them from locking arms with al Qaeda or elements of ISIS," said Sen. Chris Murphy, a Democrat from Connecticut.
But Sen. Claire McCaskill, a Democrat from Missouri and a member of the Armed Services Committee, said the U.S. intelligence community will play an important role in vetting the rebels.
She also said training could take up to a year before arms are provided.
"All of those people criticizing this choice, I have yet to hear their better idea," McCaskill said.
Sen. Collins said she's worried that it will be difficult to vet the "so-called moderate Syrian opposition."
"We spent billions of dollars and a decade training the Iraqi security forces. And look what happened when they were confronted with the ISIS threat -- they basically cut and run, with the exception of the Kurdish forces in the north."
French air support
The Obama administration has spoken of a "broad" coalition of 40 nations that will ban together to fight ISIS.
French President Francois Hollande said Thursday that there have been French reconnaissance flights over Iraq this week. When French officials "identify targets, we will act" in a short time frame, he told reporters in Paris. Hollande thanked the United Arab Emirates for allowing France to use an air base there.
Hollande stressed that France "will not go beyond" air support and will not send ground troops into Iraq.
Obama said the same for the United States on Wednesday.
"As your commander in chief, I will not commit you and the rest of our armed forces to fighting another ground war in Iraq," Obama told troops at MacDill Air Force Base in Florida.
British imams call ISIL 'un-Islamic fanatics'
In the United Kingdom on Thursday, dozens of imams signed an open letter, published in The Independent, a British newspaper, calling for ISIS to release Alan Henning. He is the latest British hostage that ISIS has threatened to kill.
If the aid worker who went to Syria to help people is not released, ISIL will be committing the "worst condemnable sin" against Islam, they write.
The letter condemns the terror group, calling ISIL "un-Islamic fanatics."
"This is not Jihad," it read, "it is a war against all humanity."
Three years of waiting
Some Syrian rebels who have been trying to oust al-Assad, have been asking the West for help since 2011. Back then, al-Assad's forces cracked down on peaceful political protests. The violence spiraled into an armed uprising that grew more complex over time and included a mix of foreign fighters.
More than 190,000 people have been killed in the Syrian war, according to an August report by the Human Rights Data Analysis Group. The United Nations commissioned the report.
The United States has provided $2.9 billion in humanitarian aid to Syria, but it has stopped short of giving Syrian rebels weapons.
The difference now? ISIS, its bloody takeover of stretches of Iraq and Syria, and its threat to Americans.
A former chief of staff for the rebel Free Syrian Army is now more concerned about ISIS than the Syrian regime.
"At this time, it is more dangerous than the regime itself," Gen. Salim Idriss told CNN's Chris Cuomo.
Idriss acknowledged there are problems among rebel forces -- which is why U.S. training would help, he said.
"They are not unified, not organized, not working according to chain of command and control," he said. "We would like now to organize them with the help of our friends in the international community to build a kind of regular army to get better results fighting the regime and other extremist groups in Syria."
Idriss said the Free Syrian Army now has about 4,000 to 5,000 "well-trained fighters" in the country.
"And if we are going to train about 5,000 more, the whole number of the moderate fighters in the FSA will be ready and capable of fighting against ISIL."
Iraq: More than 1,000 soldiers MIA from June attack
Just as in Syria, the crisis in Iraq continues to unfold.
Iraq's Human Rights Ministry said Thursday that 1,095 Iraqi soldiers still are missing after an ISIL attack in June on a military base formerly known as Camp Speicher.
ISIL says it killed 1,700 Iraqi troops in the attack. The Iraqi government hasn't released a number of those killed; Human Rights Watch says ISIS executed hundreds of soldiers.
CNN's Elise Labott, Deirdre Walsh, Barbara Starr, Ted Barrett, Jessica Moskowitz, Jim Acosta and Mohammed Tawfeeq contributed to this report.