Honduras followed suit late Monday night with President Ricardo Maduro announcing the pullout of his 370 troops "in the shortest time possible," confirming U.S. fears.
In Madrid on Monday, Spanish Defense Minister Jose Bono said Spain would pull its 1,300 troops out of Iraq within six weeks, about a month earlier than scheduled.
Brig. Gen. Mark Kimmitt sought to allay fears about the implications of the Spanish pullout, saying there would be no "security vacuum in that area at any time."
"Numerically those are numbers (the Spanish contingent) that should be able to be replaced in fairly short order," Kimmitt said.
But it remains unclear who will take the place of departing troops. The 9,500 international peacekeepers under Polish command are charged with the south-central sector, where followers of radical Shiite cleric Muqtada al-Sadr are waging a bloody rebellion.
In other developments:
Spanish Prime Minister Jose Luis Rodriguez Zapatero announced the pullout just hours after his Socialist government was sworn in, fulfilling a campaign promise. Spain is the third-largest contributor of troops to the multinational force and the sixth-largest overall in Iraq.
Zapatero had initially pledged to remove the troops if the United Nations did not take political and military control of the situation in Iraq by June 30. In making his announcement to remove them as soon as possible, Zapatero said there were no signs the situation would have changed enough to satisfy Spain by that deadline.
His decision was a setback for the Bush administration, which has been eager to portray the effort in Iraq as an international cause even though it is dominated by 130,000 American troops.
Aside from the U.S. and multinational forces, some 12,000 British troops and 2,700 Italians operate in the far south.
President Bush scolded Zapatero for the abrupt withdrawal, telling him in a telephone conversation Monday to avoid actions that give "false comfort to terrorists or enemies of freedom in Iraq."
Polish officials said they thought greater United Nations involvement might help wavering countries make new troop commitments or at least follow through with what they have already promised.
"A U.N. resolution would be a great help," Polish Defense Minister Jerzy Szmajdzinski told Poland's TVN24.
Poland has the most troops, 2,400, in the 23-nation force, and Szmajdzinski said it could not send any more.
But Albania immediately said it was ready to increase its presence. At the moment Albania's commitment is mostly symbolic, consisting of 71 non-combat troops patrolling the city of Mosul under U.S. command.
Ukraine, Australia, Portugal, Slovakia, San Salvador and the Dominican Republic said their commitments to the force would not waver.
In Tokyo, top Japanese military officials said Monday that greater U.N. involvement would make it easier for Japanese troops to carry out their humanitarian mission.
The U.S. military has been fighting on two fronts this month — in Fallujah and against the rebel Shiite cleric's militia in Najaf — sparking the worst violence in Iraq since Saddam Hussein's fall.
Since April 1, at least 1,100 Iraqis — including civilians, insurgents and security forces — have been killed, according to an Associated Press count compiled from hospital reports, Iraqi police officials and U.S. military statements. At least 99 U.S. troops have been killed in action, surpassing the deadliest full month since the U.S.-led invasion began in March 2003.
Fallujah has been largely quiet for the past few days, with only sporadic clashes.
Before dawn on Tuesday, gunmen in Fallujah opened fire on a Marine patrol near the Euphrates river, Capt. Jamie Edge said. Marines and gunmen exchanged fire for about five minutes, Edge added. There were no immediate reports of casualties.
"I think this is something we'll continue to deal with, regardless of what the security situation ends up being in Fallujah," Edge said.
Iraqi forces allied with the U.S.-led coalition are expected to begin gathering the weapons in the coming days — if anyone steps forward to surrender them.
U.S. officials have questioned how much influence Fallujah leaders have with the guerrillas.
Small groups of armed and uniformed Iraqi police and civil defense members patrolled the streets Monday for the first time since the U.S. siege began April 5. Some Fallujans emerged from their homes.
The standoff against radical cleric Muqtada al-Sadr outside Najaf has largely been on hold.
U.S. Col. Dana J.H. Pittard said there were no plans for the time being to make a move against al-Sadr in the holy city, a move moderate Shiite clerics have warned would spark outrage. Some 2,500 U.S. soldiers were deployed to Najaf, but that number was to drop to about 500, he said.