At a joint press conference with British Prime Minister Tony Blair, Mr. Bush was asked if he had said he would start bringing U.S. troops home starting next year.
"What I said is that we'll match the security needs with the number of troops necessary to secure Iraq. And we're relying upon our commanders on the ground to make those decisions," the president said. "We could have less troops in Iraq, we could have the same number of troops in Iraq, we could have more troops in Iraq, whatever is necessary to secure Iraq."
Hours later, a throng of at least 100,000 demonstrators moved through the heart of London to send an anti-war message to Mr. Bush, erecting a towering effigy of the president so it could be toppled.
The Bush administration has a plan to reduce the number of U.S. troops in Iraq from the current level of about 135,000 to around 105,000 by May.
Iraqi security forces are supposed to take up part of the load. The Washington Post reports the U.S. civilian authority in Iraqi, which disbanded the Iraqi military months ago, is now struggling to reconstitute parts of it.
Even if Iraqis step in to take the load, more Americans may head to the region. In order to replace U.S. troops rotating home, the Defense Department Wednesday alerted 15,000 reservists and National Guard members that they may be sent to Iraq. According to The New York Times, that means 58,000 troops are now standing by to head there.
In other developments:
Mr. Bush and Blair stood united on the war on terror. Blair, who has seen his approval ratings sink amid broad opposition here to the war, called the process of ensuring a stable, democratic Iraq "an essential part in defeating this fanaticism and extremism" that is killing innocents in attacks around the world.
"Our response is not to flinch or give way or concede one inch," he said. "We stand absolutely firm until this job is done."
"They need to be stopped and we will stop them," Mr. Bush said. The men spoke only hours after two bombs killed at least 27 people in Istanbul in Turkey's deadliest terrorist bombing ever.
Thursday violence continued a pattern this week of attacks against people assisting the U.S. occupation.
Jalal Johar, an official with the Patriotic Union of Kurdistan, said the powerful explosion in Kirkuk occurred about 10:30 a.m. It shattered windows and damaged doors at the two-story, yellow-and-green PUK building and blew out the windows of the nearby radio and television station.
The PUK is a group that supports American efforts in Iraq. Party chief Jalal Talabani is the current head of the U.S.-installed Iraqi Governing Council. One of the PUK's regional rivals in the Kurdish area, Ansar al-Islam, is believed to have ties to al Qaeda. No group claimed responsibility for the attack.
Some U.S. officials suspect Ansar al-Islam is working with Saddam Hussein loyalists, including Izzat Ibrahim al-Douri, former vice chairman of the Revolutionary Command Council. U.S. officials have offered a $10 million reward for the capture of al-Douri.
Late Wednesday, a car bomb exploded outside the home of Sheik Amer Ali Suleiman, a tribal leader in Ramadi, about 60 miles west of Baghdad inside the "Sunni Triangle" where anti-U.S. attacks are concentrated. Hospital workers said Thursday that two people were killed.
Suleiman is a leader of the Duleim tribe, one of the largest Sunni Muslim tribes in Iraq. He also is a member of the city council and is close to the Americans.
Rebels repeatedly have attacked police stations and Iraqis perceived to be cooperating with the occupation.
In Basra, the Assyrian Democratic Movement said its representative on the municipal council, Sargoun Nanou Murado, was abducted Tuesday on his way to work. His body was found Wednesday, a statement said.
The Assyrian Democratic Movement, which represents Iraq's Assyrian minority, is represented on the 25-seat Governing Council.
The assassination is the second this week of people working with coalition authorities in southern Iraq. In the town of Diwaniyah, gunmen on Tuesday killed the education ministry's director general for that province.