U.S. launches military operation against al-Shabaab group in Somalia

(CNN) -- U.S. military forces conducted an operation Monday against the al-Shabaab network in Somalia, Pentagon press secretary Rear Adm. John Kirby said.

"We are assessing the results of the operation and will provide additional information as and when appropriate," he said.

No other information was immediately available.

The United States designated al-Shabaab, an al-Qaeda-linked militant group, as a foreign terrorist organization in March 2008.

News of the operation comes just one day after al-Shabaab fighters dressed in government military uniforms approached the gate of a high-security prison in Mogadishu, set off a car bomb and fired their way into the building.

During the assault on the National Intelligence and Security Agency prison, three security guards and two civilians were killed and 15 others were injured, Security Ministry spokesman Mohamed Yusuf told CNN.

The NISA prison is underground and is close to the Somali presidential palace in Mogadishu. Hundreds of inmates, mostly al-Shabaab members, are being held there.