The US senate's foreign relations committee chairman John Kerry has come to Afghanistan to meet the chief US and Nato commander General Stanley McChrystal who has recommended a radical change in US strategy there.
Kerry, in Afghanistan on Thursday, said he had many questions for McChrystal, who has asked for at least 40,000 more troops to fight the war there.
"I don't know the answer to that question ... I am very wary of it because of past experience and because of some of the challenges that I see," Kerry said in response to McChrystal's recommendation.
He said neither of the two extremes - a nationwide counterinsurgency and nation-building effort in Afghanistan nor "walking away from the place" - were do-able, Aljazeera.net reports.
It was also reported, officials said they would not require the 40,000-troop increase preferred by Army Gen. Stanley A. McChrystal, the U.S. and allied commander. A number of White House officials favor sending fewer than 20,000 additional troops.
In February, President Obama approved 21,000 additional American troops, bringing the U.S. force to 68,000. There also are about 38,000 North Atlantic Treaty Organization troops in the country, The Los Angeles Times reports.
In the meantime, Mr Brown said he was prepared, in principle, to send an extra 500 troops to Helmand Province in southern Afghanistan - where Nato troops are fighting fierce battles with the Taliban - in response to military advice.
He said the reinforcements would happen only if troops had the equipment and training necessary to perform the tasks asked of them.
Given the conditions placed on their deployment, the BBC's defence correspondent Caroline Wyatt said it was unlikely the extra troops would be in Afghanistan soon.
But the announcement did send a clear message to the US as it reviews its Afghan strategy, she added.
Mr Obama has been considering future troop options after a request by Gen McChrystal, BBC News reports.