WASHINGTON - US Defense Secretary Robert Gates said Thursday he expects US troop level in Iraq to continue to come down if conditions there improve, as the top US military officer warned of a growing challenge in Afghanistan.

Gates, however, said he did not think it was possible to reduce US force levels to 100,000 by the end of the year as he had once hoped.

His comments appeared to put him at odds with General David Petraeus, the US commander in Iraq, who called for an indefinite pause in troop reductions after the last of the US "surge" brigades are withdrawn at the end of July.

President George W. Bush embraced Petraeus's recommendations in a speech earlier Thursday in which he said the general "will have all the time he needs" before make further reductions in US forces.

But Gates told the Senate Armed Services Agreement there would be a "brief pause for consolidation and evaluation."

"I do not anticipate this period of review to be an extended one, and I would emphasize that the hope, depending on conditions on the ground, is to reduce our presence further this fall," he said.

"If the conditions continue to improve in Iraq, as we have seen them improve for the last 14 or 15 months, then we believe the circumstances are in place for him (Petraeus) to be able to recommend continuing drawdowns," he said.

"But while I think we have used different words, that is my understanding and my expectation," he said.

The withdrawal of five surge brigades will reduce the size of the US force in Iraq from over 150,000 troops today to about 140,000 -- more than before the surge began in early 2007.

Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, warned that while Iraq is the US military's most pressing priority, it is not the only one.

"I need the rest of our military focused on the rest of our challenges, which are, in this dangerous world, many and formidable," he said.

Mullen said he was "deeply concerned" about Afghanistan, where the United States recently deployed 3,500 additional marines to help fill growing gaps in command requirements.

"The Taliban is growing bolder, suicide attacks are on the rise, and so is the trade in illegal narcotics," Mullen said.

"In this economy of force operation, we do what we can. But doing what we can in Afghanistan is not doing all that we should," he said.

jm/sg

04/10/2008 19:55 GMT