WASHINGTON (Reuters) - President Bush on Thursday turned to his closest allies in an effort to set a deadline for Iraqi disarmament, as his spokesman said the U.S.-led diplomatic endgame would last "weeks, not months."

White House spokesman Ari Fleischer also showed support for a reported Saudi proposal for Iraqi President Saddam Hussein to leave office voluntarily, saying exile would be a "helpful" way to resolve the showdown peacefully.

The proposal was expected to be discussed at Bush's meeting with Saudi Foreign Minister Saud al-Faisal later on Thursday.

Bush was also expected to discuss a possible deadline for Iraq to comply with U.N. disarmament demands in a separate meeting with Italian Prime Minister Silvio Berlusconi, a supporter of his Iraq policy.

"The president is using this window now to engage in very busy and active diplomacy. This will take place in a period of weeks, not months," Fleischer told reporters.

Asked about the prospects of an exile deal for Saddam, he said, "If Saddam Hussein were to go into exile, that would be a very helpful event for the world. ... The more pressure on Saddam Hussein and the more unified the world is the more the likelihood of this being resolved peacefully, as a result, for example, of his going into exile."

The spokesman said Bush had not given up hope of averting war, but reiterated the White House's determination to disarm Iraq by force if necessary.

Bush got much needed diplomatic support from eight European leaders, who appealed for unity in a joint opinion article published in several newspapers in Europe and America.

The article highlighted a split in Europe over Iraq -- in which big powers France, Germany and Russia remain strongly opposed to an early decision on war.

But Fleischer said Bush was confident "that if the call is made, European nations, European leaders will answer."

The article was signed by British Prime Minister Tony Blair, Berlusconi, Spain's Jose Maria Aznar, Portugal's Jose Barroso, Denmark's Anders Fogh Rasmussen, the Czech Republic's Vaclav Havel, Poland's Leszek Miller and Hungary's Peter Medgyessy.

"The transatlantic relationship must not become a casualty of the current Iraqi regime's persistent attempts to threaten world security," the eight leaders wrote.

"The Iraqi regime and its weapons of mass destruction represent a clear threat to world security," they said.

Only five of the 15 existing EU leaders signed the letter. Dutch Prime Minister Jan Peter Balkenende refused, a spokesman said.

With thousands of U.S. troops pouring into the Gulf region, administration officials say Bush and his top aides are opening a final "diplomatic window" with allies in a last-ditch attempt to avert a seemingly inevitable war with Iraq.

In addition to Berlusconi, who has told the United States that transport planes bound for the Gulf can use military bases in Italy for stopovers and refueling, Bush also meets on Friday with Blair, his strongest supporter on Iraq.

"They will discuss the next steps to take in the face of Iraq's failure to date to disarm and comply with United Nation's resolution 1441," said White House National Security Council spokesman Sean McCormack.

Prince Saud, the highest ranking Saudi official to visit Washington in months, is expected to focus mostly on Iraq and U.S. military preparations for a possible war.

A major element of the diplomatic effort will be Secretary of State Colin Powell's appearance before the U.N. Security Council next week.

Bush said Powell would lay out the U.S. case that Saddam has been concealing weapons of mass destruction from inspectors and consorting with what he called terrorist groups, including the al Qaeda network of Osama bin Laden blamed for the Sept. 11, 2001, attacks on the United States.

Iraq denies the allegations.

The Bush administration is struggling to convince reluctant allies and much of the American public that Saddam is an imminent threat requiring use of force. Bush has tried to establish a connection between Iraq and bin Laden's militant network to bolster his case.