"Obviously, the British people and those that serve there have borne an enormous price in casualties. Obviously, we're thankful for a strengthening of the coalition," Mr Gibbs said.
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, the commander of international forces in Afghanistan, for what has been widely reported as up to 40,000 more Nato troops.
Isaf's major combat teams in Afghanistan
Isaf's provincial reconstruction teams in Afghanistan
Over 40 countries contribute
forces to the international mission in Afghanistan. Full details on the
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