US President George W Bush said Monday would be a "moment of truth for the world", following a summit on the Iraq crisis with his Spanish and British allies in the Portuguese Azores islands.
Iraq says it is prepared for war
The United Nations Security Council faces a Monday night deadline to agree a new resolution demanding Saddam Hussein's immediate disarmament and thus avert a possible US-led war.
With a declaration of war possibly hours away, the Security Council is scheduled to hold a closed door meeting at 1000 local time (1500 GMT) on Monday, a UN spokesman said.
The BBC's Washington correspondent Justin Webb says if it quickly becomes obvious that no progress can be made, President Bush could address the American people on Monday night.
Our correspondent says Mr Bush's speech will not be an immediate declaration of war, but it will give Saddam Hussein only a few further hours to leave Iraq or face an attack.
Iraq has reacted defiantly to President Bush's ultimatum, saying a war would be fought all over the world.
The UN's chief weapons inspector Hans Blix was preparing to submit a programme to the Security Council for the completion of the disarmament process in Iraq.
Dr Blix called the situation "very threatening", but he said that unless there was a sudden decision to withdraw the UN team from Iraq, inspectors would continue to work as normal.
The United States, meanwhile, has ordered non-essential diplomats and their families to leave Kuwait, Israel and Syria because of the threat of conflict with Iraq.
The UK on Monday advised all Britons, except diplomatic staff, to leave Kuwait as soon as possible.
As speculation about an imminent attack grew, the UN ceased its monitoring operations in a demilitarised zone separating Iraq and Kuwait, a UN official in Kuwait said.
The 200-km (120-mile) wide zone - which US forces would have to cross in any invasion of Iraq - was set up after Iraqi forces were repelled from Kuwait in the 1991 Gulf War.
The Azores summit represented a final push to try to obtain enough support among Security Council members to make a vote on a new resolution worthwhile.
France has shown its card
French President Jacques Chirac repeated that his country would veto any UN resolution which paved the way for war on Iraq, telling a US television network "we should pursue [diplomacy] until we've come to a dead end".
After weeks of diplomatic wrangling, the US, Britain and Spain, who need nine sympathetic votes on the 15-seat Security Council, have only managed to elicit the support of council member Bulgaria.
At a press conference following the Azores summit, President Bush made his frustration with France clear.
"We have an expression in Texas that says, 'Show your cards'," he said.
"France has shown its card. Now we have to see tomorrow [Monday] what that card meant."
UK Prime Minister Tony Blair also made "a final appeal for there to be that strong unified message on behalf of the international community" to Saddam Hussein in what correspondents say was aimed at the French.
US forces in the Gulf now top what our Pentagon correspondent calls the "magic number" of 250,000 personnel and both US and UK forces say they are ready for any military assault.
Saddam Hussein appeared prepared for war, warning that if Iraq is attacked the battle will be "wherever there is sky, land and water in the entire world."
Iraqi Foreign Minister Naji Sabri told an Arabic television station that tens of thousands of Iraqi men and women were ready to be martyrs for any war against the "treacherous" American enemy.
After an hour of talks, the three leaders emerged not only to deliver their ultimatum but to point to their vision of a post-Saddam Iraq.
Mr Blair pledged that in the event of war, Iraq's territorial integrity would be protected and the country's natural resources would be used for the benefit of the Iraqi people.
His Spanish counterpart, Jose Maria Aznar, said they were willing to make one last effort for peace, but that if Saddam Hussein wanted to prevent a war, he should do it now by disarming.
Mr Bush said he would go back to the UN to seek co-operation on rebuilding Iraq if war is declared.
"We're committed to a goal of a united Iraq with democratic institution in which members of all ethnic and religious groups are treated with dignity and respect," he said.