The US vetoed a UN Security Council draft resolution that sought to condemn an Israeli military offensive in the Gaza Strip and demand Israeli troops pull out of the territory.
US Ambassador John Bolton said the Arab-backed draft resolution rejected on Saturday was "biased against Israel and politically motivated."
"This resolution does not display an evenhanded characterization of the recent events in Gaza, nor does it advance the cause of Israeli-Palestinian peace to which we aspire and for which we are working assiduously," he told the Security Council.
The veto unleashed a flurry of criticism from some countries in the Middle East.
"This decision by the US government gives unlimited cover to commit more massacres of innocent Palestinians," spokesman for the Hamas-led Palestinian government Ghazi Hamad said.
"This is a shame on the American administration, which says it is trying to promote human rights and democracy in the Middle East," he added.
US Secretary of State Condoleezza Rice said, however, that the draft resolution was not acceptable because it contained "inflammatory and unnecessary language" and thus would have aggravated the situation in Gaza.
"We do not believe the resolution was designed to contribute to the cause of peace," she said in a statement.
The draft received 10 votes in favor and four abstentions, along with the US vote against. Britain, Denmark, Japan and Slovakia all abstained. The US is one of five permanent members of the council that have the power to veto resolutions.
It was the second US veto this year of a Security Council draft resolution concerning Israeli military operations in Gaza. The US blocked action on a document this summer after Israel launched its offensive in response to the capture of an Israeli soldier by Hamas-linked Palestinian militant forces.
Palestinians strengthened calls for Security Council action after an early morning Israeli artillery barrage in the northern Gaza town of Beit Hanoun killed 19 people on Wednesday.
In an open session of the General Assembly on Thursday, Palestinian UN observer Riyad Mansour called the attack "state terrorism" and said the perpetrators should be held accountable under international law for war crimes.
Israel has expressed regret for the loss of life in Beit Hanoun but said it will continue operations to stop militants from launching rockets into Israel. Prime Minister Ehud Olmert was scheduled to visit Washington yesterday to meet with US President George W. Bush.
Qatar's UN Ambassador Nassir Al-Nasser warned in the Security Council meeting that the failure of the body to act on the draft would lead to more Israeli violence against Palestinians.