US President Barack Obama may announce a substantial surge in US troops to Afghanistan, the BBC has been told.
The BBC’s Newsnight said the increase could be announced next week.
However, White House press secretary Robert Gibbs dismissed the report, saying the president had not yet made a decision on troop numbers.
Earlier, Prime Minister Gordon Brown said Britain would send 500 more forces personnel to Afghanistan – but only if key conditions were met.
They would be sent as long as they had the necessary equipment, if other Nato allies boosted their troop numbers and more Afghan soldiers were trained.
There are currently about 9,000 UK personnel in Afghanistan.
Newsnight said the Obama administration had already told the UK government it would soon announce a substantial increase to its military forces in Afghanistan.
The programme said it understood an announcement could come next week, in time for a Nato defence ministers meeting in Bratislava.
On the programme, Said Jawad, the Afghan ambassador to Washington, was asked if he could confirm that the United States would be sending more troops.
“We should wait for the official announcement by the US government but all indications are that President Obama is honouring the request by Gen McChrystal.”
He was asked if that meant 40,000 to 45,000 extra troops.
“This is part of the request, yes,” he said.
But Mr Gibbs said the president had “not made a decision”.
He added: “I think that you can assume that the BBC will not be the first outlet for such a decision.”
However he did say the US had been “coordinating our review with our allies” and that Mr Brown had “communicated to us their decision to send more troops” in a phone call last week.
“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.