The U.S. troop "surge" in Iraq that President George W. Bush ordered last year has ended after the last of five additional combat brigades left the country, a U.S. military spokesman said on Tuesday.

The remaining troops from that brigade departed over the weekend, leaving just under 147,000 American soldiers in Iraq, the spokesman said.

"The final elements of the surge brigade have now left, getting out a few days ahead of schedule," he said.

The U.S. military had 20 combat brigades in Iraq at its peak in 2007, with troop levels around 160,000-170,000.

The current number is well above the 130,000 troops in Iraq when Bush ordered the deployment in January 2007. The Pentagon said last February it expected 140,000 troops to be in Iraq once the five brigade drawdown had finished.

The military spokesman said troop numbers fluctuate in general, with replacements in Iraq at the same time as forces they were relieving.

"You don't necessarily get a one-for-one swap when a new brigade relieves one that is leaving - in some instances, some of the arriving brigades have been considerably larger than the brigades they replaced," he said.

Bush sent 30,000 extra soldiers to Iraq last year to quell sectarian violence between majority Shi'ites and minority Sunni Arabs that threatened to tip the country into all-out civil war.

U.S. officials say the buildup helped cut violence in Iraq to four-year lows. Other factors were a decision by Sunni Arab tribal leaders to turn against al Qaeda and a ceasefire by anti-American Shi'ite cleric Moqtada al-Sadr, whose Mehdi Army militia was accused of carrying out sectarian killings.

In late May, the U.S. commander in Iraq, General David Petraeus, said he expected to recommend resuming withdrawals after a 45-day freeze to take stock of security conditions once the last reinforcement brigade had left.

U.S. troop levels are a key battleground in November's U.S. presidential election.

Democratic contender Barack Obama, who is visiting Iraq, has pledged to remove U.S. troops within 16 months of taking office should he win the election.

Republican candidate John McCain on Monday appeared to leave the door open to a large-scale drawdown of U.S. troops in the next two years if conditions on the ground were suitable, saying success had made it possible for troops to return home.

McCain has long argued against setting a timetable for a U.S. troop withdrawal.