The US says it will help to investigate the car bomb which killed a top intelligence official in Lebanon.
Secretary of State Hillary Clinton and Lebanese PM Najib Mikati agreed to the joint probe in a phone call, said a US spokeswoman.
Wissam al-Hassan and at least two other people were killed in Friday's bombing.
Clashes erupted after his funeral on Sunday, as protesters called on Mr Mikati to quit. At least three people were killed overnight in Tripoli.
In Beirut, police fired warning shots and tear gas as some demonstrators tried to storm the government offices in the Lebanese capital.
There were reports of further violence in southern and western Beirut overnight.
Opposition figures have blamed neighbouring Syria for the attack.
Protests have been held against Syria and its Lebanese allies, amid fears the conflict there could spill across the border.
Mr Hassan, 47, was close to the 14 March opposition and the Hariri family, part of the anti-Syrian opposition.
The Syrian government condemned the attack, which also killed one of Mr Hassan's bodyguards and a woman nearby. Earlier reports said as many as eight people died.
On Sunday, Mrs Clinton stressed "the United States' firm commitment to Lebanon's stability, independence, sovereignty and security," State Department spokesperson Victoria 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," the statement added.
Mr Hassan led an investigation into the assassination of former Prime Minister Rafik Hariri, which implicated Damascus.
He also recently organised the arrest of a former minister accused of planning a Syrian-sponsored bombing campaign in Lebanon.
Syrian troops withdrew from Lebanon in 2005 after a 29-year-long presence, in the wake of Mr Hariri's killing.
But the BBC's Wyre Davies reports that there are concerns in Beirut that Damascus is able to reach into Lebanese society both directly and through its allies.
Mr Hassan was buried next to Mr Hariri on Sunday.
Many mourners waved the light blue flag of the Sunni-based opposition Future Party, while others carried Lebanon's national flag.
Mr Mikati says he offered to resign after the attack, but accepted a request from President Michel Suleiman to stay on in order to avoid a power vacuum.