Washington - US President George W Bush on Thursday endorsed his top commander in Iraq's recommendation to halt US troop withdrawals once the surge ends in July, leaving open the possibility that 140,000 soldiers will still be there when he leaves office.

Bush offered his strong support to General David Petraeus, who called for a 45-day pause in troop reductions after the buildup ends to assess the effect of the increased US presence that was ordered last year by the president to quell violence and sectarian killings.

"I have told him he'll have all the time he needs" to determine whether additional cuts can take place, Bush said.

Bush, acknowledging the strain on the US military, said he was ordering a 12-month limit to deployments in Iraq, reducing the length of troop rotations from 15 months.

Bush's speech came after Petraeus, along with ambassador Ryan Crocker, on Wednesday completed two days of testimony before Congress to outline the progress that has been made in Iraq since the troop surge came into full effect last summer.

Bush said that under the troop buildup the US and Iraqi governments have regained the initiative, sectarian violence has dropped and that al-Qaeda was on the defensive and "we're now working to deliver a crippling blow."

"With the surge, a major strategic shift has occurred," Bush said.

Petraeus said during his testimony before Senate and House committees that it was still too early to conclude whether he will advocate additional cuts in the US presence once the 45-day pause ends.

Congressional Democrats, who opposed the surge and want a more rapid timeframe for withdrawing troops, have said Petraeus' plan amounts to an endless US commitment to Iraq and a government in Baghdad that has not shown a willingness to take responsibility for security or tackle thorny political issues.

"Our troops remain in this endless, endless intractable war," Senate Majority Leader Harry Reid said, calling Bush's strategy "one step forward and two back."

Bush is set to leave office in January, meaning his successor will most likely inherit the sizeable US contingency in Iraq and a war viewed disfavourably by the American public.

Bush, speaking from the White House, said failing in Iraq would embolden Iran, serve as a victory for al-Qaeda and other extremist groups, and jeopardize US security and prestige.

"If we fail there, al-Qaeda would claim a propaganda victory of colossal proportions and they could gain safe havens in Iraq from which to attack the United States, our friends and our allies," Bush said.

"This would diminish our nation's standing in the world and lead to massive humanitarian casualties and increase the threat of another terrorist attack on our homeland," he added.

Bush also issued a stark warning to Iran, which he said plays a "destructive" role in Iraq. He urged Tehran to make the "right choice" to stop arming and training militias in Iraq or "America will act to protect our interests and our troops and our Iraqi partners." (dpa)