The United States said it was taking measures to protect its citizens worldwide after protesters angry about an online film considered offensive to Islam attacked U.S. diplomatic compounds in Libya and Egypt Tuesday.

In Libya, witnesses say members of a radical Islamist group called Ansar al-Sharia protested near the U.S. Consulate in Benghazi, where NATO jets established no-fly zones last year to halt ground attacks from then-Libyan strongman Moammar Gadhafi.

The group then clashed with security forces in the city, blocking roads leading to the consulate, witnesses said.

The U.S. ambassador to Libya, J. Christopher Stevens, was killed in the attack, the State Department said. Sean Smith, a Foreign Service information management officer, and two other U.S. personnel also died in the violence in Benghazi, Secretary of State Hillary Clinton said in a statement Wednesday.

"Our hearts go out to all their families and colleagues," Clinton said.

"All the Americans we lost in yesterday's attacks made the ultimate sacrifice. We condemn this vicious and violent attack that took their lives, which they had committed to helping the Libyan people reach for a better future," she added.

In an earlier statement, Clinton said she condemned the attack on the U.S. facilities "in the strongest terms" and that following Tuesday's events, the U.S. government was "working with partner countries around the world to protect our personnel, our missions and American citizens worldwide."

Libya's General National Congress also condemned the attack in Benghazi, saying it "led to the regrettable injury and death of a number of individuals." Lawmakers said in a statement Tuesday night that they were investigating.

It was unclear whether the two attacks were coordinated, CNN national security contributor Fran Townsend said Tuesday night.

"One such breach of an embassy or consulate's walls or security on any given day would be tremendous news. ... The fact that two of them happened on the same day that is the 9/11 anniversary where Americans are remembering those that we lost, you have to ask yourself, what are American officials trying to understand about this and whether or not these two are related?" she asked.