US Defence Secretary Robert Gates has hinted at bigger cuts to troop numbers in Iraq than those so far approved by President George W Bush.
Mr Bush wants the troops' role to become more advisory
Mr Gates suggested the current level of more than 160,000 soldiers could be cut to about 100,000 by the end of 2008.
Mr Bush said on Thursday about 30,000 troops might return by next summer.
Mr Gates spoke as a White House report suggested Iraq's government has made little progress in meeting key military and political benchmarks set by the US.
Both Mr Gates and Mr Bush stressed that any reduction in troop levels would be entirely dependent on the success of their mission.
The defence secretary said: "The whole situation and recommendations at this point are based on an analysis by the commander in the field, plus... the situation on the ground.
"One of the sad aspects of war is there is no script. That history hasn't been written yet. And the enemy has a vote."
On Thursday night the president used a prime-time televised address to outline plans for a gradual withdrawal from Iraq and a transition to a more advisory role for US troops there.
He said he had taken the advice of the US commander in Iraq, Gen David Petraeus, who had given his own progress report to Congress earlier this week and said the recent military "surge" in Iraq was working.
The White House report, which is the Bush administration's own assessment of the situation in Iraq, says Iraq has performed satisfactorily on nine out of 18 benchmarks - one more than in July.
Among the failures, it cited militia control over security forces and a failure to enact laws on sharing oil resources.
Democrats said the report showed the president's current policies were failing.
"As hard as they may have tried to spin it, today's assessment by the White House... once again shows that the president's flawed escalation policy is not working," Senator Harry Reid said.
The White House said US efforts in Iraq extended far beyond the 18 benchmarks, adding that many of the objectives were being met even if the formal benchmarks had not been reached.