The UN Security Council has agreed to a request from Lebanon to help investigate the murder of leading anti-Syrian politician Pierre Gemayel.
A vigil at the site of Mr Gemayel's killing, ahead of Thursday's funeral
The council's current president said the decision was taken quickly and needed no discussion.
The UN commission already looking into the murder of ex-Lebanese PM Rafik Hariri in 2005 will take on the probe.
Many in Lebanon accuse Syria of involvement in Mr Gemayel's death but Damascus rejects the claims.
Mr Gemayel, Lebanon's industry minister, was shot in his car in a Christian area of Beirut on Tuesday.
Crowds have gathered in Mr Gemayel's village for his funeral.
On Wednesday US President George W Bush called Lebanese Prime Minister Fouad Siniora to pledge support for Lebanon's independence from what he called the "encroachments of Iran and Syria".
The Security Council's announcement came shortly after UN Secretary-General Kofi Annan expressed his concern over events in Lebanon and said he had spoken to Mr Siniora.
The BBC's UN correspondent in New York, Laura Trevelyan, says the council had agreed on the day of Mr Gemayel's murder to set up an international tribunal to try the killers of Mr Hariri once they are brought to justice.
She says the tribunal could also eventually try Mr Gemayel's killers but such a tribunal is highly controversial in Lebanon.
Pro-Syrian politicians are against it, she says, and it cannot begin to be set up until the Lebanese parliament votes for it.
Mr Gemayel's killing has created a political crisis in Lebanon - the death or resignation of two more cabinet ministers would bring down the government.
The Security Council's quick decision followed calls from US Ambassador John Bolton for rapid action "while the crime scene evidence is still fresh".
The UN inquiry already in place, led by Belgian prosecutor Serge Brammertz, is also looking into another 15 apparently politically motivated attacks, including Mr Hariri's death.
In his telephone call to Mr Siniora, Mr Bush reiterated the "unwavering commitment of the United States to help build Lebanese democracy," an official at the White House said.
Mr Bush has not specifically blamed Iran or Syria for Mr Gemayel's murder but he has called for a full investigation to identify "those people and those forces" behind the killing.
Lebanon is holding three days of official mourning for Mr Gemayel.
A huge crowd of mourners accompanied Mr Gemayel's coffin as it arrived in his home village of Bikfaya, east of Beirut, on Wednesday.
Mr Gemayel's supporters have called for a mass turnout at his funeral, and there is a large military presence both in the village and in Beirut.