Secretary of State Condoleezza Rice meanwhile made a hasty telephone call to Lebanon's Prime Minister Fuad Siniora, amid signs of US concern over the fate of the Beirut government which emerged from Lebanon's 'cedar revolution.'
"Today we saw again the vicious face of those who hate freedom," Bush told American troops in Hawaii during a trip home from Asia.
"We strongly condemn the assassination today in Lebanon of Pierre Gemayel."
Bush did not apportion blame but called for an investigation into "those people and those forces" behind the killing of the anti-Syrian Christian leader.
"We support the Siniora government and its democracy and we support the Lebanese people's desire to live in peace and we support their efforts to defend their democracy against attempts by Syria, Iran and allies to foment instability and violence in that important country," said Bush.
The killing came against a backdrop of global calls, so far resisted by the United States, for a dialogue with Syria and Iran over chaos in Iraq.
But Bush's tone, further bolstered in a written statement on the killing, appeared to cast further doubt on already slim chances of such a diplomatic opening.
"Syria's refusal to cease and desist from its continuing efforts to destabilise Lebanon's democratically elected government" was a violation of UN Security Council resolutions, Bush said.
"We also demand that Syria treat Lebanon as a genuinely sovereign neighbour, establishing full diplomatic relations with Lebanon," he said.
The president said the assassination made it even more important for the United Nations Security Council to make a decision on a tribunal to investigate the killing of Lebanese ex-premier Rafiq Hariri last year.
US Under-secretary of State Nicholas Burns earlier branded the killing of Gemayel, gunned down in a Beirut suburb, as an "act of terrorism" aimed at Syria's opponents in the country.
Rice called the Lebanese prime minister to express condolences and to reiterate the support of the United States for "Lebanese democracy and for the Siniora government," the State Department said.
America's ambassador to the United Nations John Bolton meanwhile pointed the finger at Syria, noting the string of assassinations of anti-Damascus
politicians in Lebanon.
"One can follow the logic there, I think," Bolton told CNN.
Syria issued its most complete denial of involvement in the killing through its embassy here.
"This charade of blaming Syria for every malicious event in Lebanon has been exposed a long time ago and is, simply, losing all credibility," the embassy statement said.
"Syria is outraged by this terrible act. In a time when the international community is advocating more engagement with Syria, such an act only stands to undermine these initiatives."
The embassy suggested the killing was timed by unnamed elements to discredit Syria ahead of the Security Council decision expected imminently on the Hariri tribunal.
"It's no coincidence that Pierre Gemayel was assassinated on the day the Security Council is discussing a Lebanese issue," the statement said.
Earlier this month, Washington issued an unusually public warning over "mounting evidence" that Iran, Syria and the Hizbullah were plotting assassinations as part of a plan to topple Siniora's government.
The United States has been a firm backer of Siniora, though tensions emerged during the Israeli war with Hizbullah in Lebanon earlier this year.
Washington has pointed to what it says is Syria's interference in Lebanon to explain its reluctance to talk directly with the government in Damascus, despite increasing pressure here and abroad for a dialogue over Iraq.
Washington withdrew its ambassador to Damascus after the Hariri assassination last year, and also accuses Syria of aiding insurgents in Iraq.

Copyright AFP (Agence France-Presse), 2006