WASHINGTON: U.S. Secretary of State Hillary Clinton spoke with Lebanese Prime Minister Najib Mikhail on Sunday to reiterate U.S. condemnation of Friday's deadly car bombing in Beirut, and the two agreed that Washington would help investigate the attack.

In a phone call with Mikati, Clinton called the attack that killed intelligence chief Brigadier General Wissam al-Hassan and others "heinous" and offered condolences, State Department spokesperson Victoria Nuland said.

"The secretary emphasized the United States' firm commitment to Lebanon's stability, independence, sovereignty and security," Nuland said in a statement.

"She noted the importance of political leaders working together at this sensitive time to ensure that calm prevails and that those responsible for the attack are brought to justice."

Nuland said Clinton and Mikati agreed that the United States would assist the investigation.

Opposition leaders and their supporters accuse Syria of being behind Friday's attack, and say Mikati is too close to Syrian President Bashar al-Assad and his Lebanese ally Hezbollah, which is part of Mikati's government.

Thousands turned out Sunday in downtown Beirut's Martyrs' Square for Hassan's funeral, which also served as a political rally. Violence erupted after an opposition leader demanded that Mikati step down to pave the way for talks on the crisis.