US President George W Bush has pledged to support Lebanon's independence from what he called the "encroachments of Iran and Syria", a US official said.
Lebanon is holding three days of mourning for Mr Gemayel
Mr Bush's promise came in a call to Lebanese Prime Minister Fouad Siniora, following Tuesday's murder of leading anti-Syrian politician Pierre Gemayel.
Many people in Lebanon blame Syria but Damascus has denied any involvement and condemned the assassination.
Crowds have gathered in Mr Gemayel's village for his funeral on Thursday.
Mr Gemayel, Lebanon's industry minister, was shot in broad daylight in his car in a Christian area of Beirut.
He was the fifth anti-Syrian Lebanese politician to be killed in the past two years, and his murder happened at a time of acute political crisis in Lebanon.
In his telephone call to Mr Siniora, Mr Bush reiterated the "unwavering commitment of the United States to help build Lebanese democracy and to support Lebanese independence from the encroachments of Iran and Syria," 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.
President Bush also "pointed out that violence and unrest in Lebanon will not stop the international community from establishing the special tribunal for Lebanon", National Security Council spokesman Gordon Johndroe said.
This refers to plans, approved on Tuesday by the UN Security Council, for an international tribunal to try suspects in the murder of former Lebanese Prime Minister Rafik Hariri.
Last week, Lebanon's cabinet endorsed the plans, despite the resignations of six pro-Syrian ministers opposed to it.
Feb 2005: Former PM Rafik Hariri
June 2005: Anti-Syria journalist Samir Kassir
June 2005: Ex-Communist leader George Hawi
Dec 2005: Anti-Syria MP Gebran Tueni
Nov 2006: Industry Minister Pierre Gemayel
A UN inquiry has implicated Syria in the killing in February 2005, although Syria has denied involvement.
Syrian officials have also vocally rejected any involvement in Mr Gemayel's murder.
"I am here as usual in the accusation box that some of the Lebanese forces are trying to throw on us," Syria's ambassador to the UK, Sami Khiyami, told the BBC.
"This killing happened in the Christian neighbourhood of Mr Gemayel by people who cold-bloodedly were armed and acted completely at ease with their environment, so I think there should be a serious investigation to find out who the killer is."
There has also been condemnation of the killing from Iran and from Hezbollah, the Shia Muslim Lebanese political and militant group.
Lebanon is holding three days of official mourning for Mr Gemayel, whose funeral is due to take place on Thursday.
Bells tolled and a huge crowd of mourners accompanied Mr Gemayel's coffin as it arrived in his home village of Bikfaya, east of Beirut.
MARONITES are Christians affiliated to the Roman Catholic Church. 800,000-900,000 live in Lebanon, roughly 25% of population. Current patriarch is Cardinal Nasrallah Sfeir. Lebanon's constitution requires that the president is Maronite
PHALANGE (Kataeb in Arabic) is a Lebanese political party advocating Maronite interests, and dominated by the Gemayel family. Its militia was a major player in the civil war
There was sombre applause from the crowd as the body passed.
Women threw rice from balconies onto the coffin, which was draped in the striped flag of his Phalange party, and there were occasional bursts of guns fired into the air.
As a priest said prayers at the Gemayel family home, the minister's friends and family wept over his coffin.
Mourners filed past, offering condolences to his father, former President Amin Gemayel.
"This is the fifth martyr for the Gemayel family. There was my brother, my nephew, my niece, a cousin and now it is Pierre, my son," Amin Gemayel said.
"It is a real tragedy, but we still have faith and, whatever sacrifices have to be made, we will still stay the course, and it is a battle that we are fighting for freedom and democracy in Lebanon."
Amin Gemayel, who took a telephone call from President Bush, said they were counting on international support to find his son's killers.
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.