Four Lebanese bystanders have been killed in a bomb blast targeting a US embassy vehicle in a northern suburb of the capital, Beirut, US officials say.
No American diplomats or citizens were in the car at the time, but a Lebanese driver working for the embassy was hurt, the US state department said.
The US Secretary of State, Condoleezza Rice, expressed "outrage" at the "terrorist attack".
Lebanon has been hit by a series of bomb attacks in recent years.
The powerful explosion shook the mainly Christian Doura area and send a thick plume of black smoke over the city.
Television footage from the scene showed damaged cars and blood stains on the street of a mixed commercial and residential area.
The bomb exploded on one of the main roads running through the north of the city.
Ms Rice, who is visiting Saudi Arabia, said the US was "outraged".
"The United States will, of course, not be deterred in its efforts to help the Lebanese people, to help the democratic forces in Lebanon, to help Lebanon resist force and interference in their affairs," she said.
A Lebanese army official said at least six people had been injured, including a non-US foreign national.
There has been no claim of responsibility so far.
The attack comes at a time of political crisis in Lebanon with rival pro-Syrian and pro-Western parties deadlocked over efforts to elect a president.
It also coincides with an extensive Middle East tour by US President George W Bush, who has been in the Gulf and is heading to Egypt, after a landmark trip to Israel and the occupied West Bank.
Lebanon's pro-Syrian former President Emile Lahoud stood down in November, and a parliamentary vote to find a successor has been postponed at least 11 times.
Most of those killed in bombings have been prominent opponents of Syrian influence in Lebanon, the most significant being former Prime Minister Rafik Hariri in 2005.
In addition to the political schism, Sunni militant groups have been waging war against Lebanon's state security forces and have openly threatened US interests.
BBC Middle East analyst Roger Hardy says whoever was responsible, they have raised the tension just a few days before the parliament tries once again to break the political deadlock.