They will reportedly be deployed later this year and come in addition to 17,000 troops already due to reinforce the 38,000 on the ground.
 

US Senator Carl Levin said the latest extra troops could be deployed in June.

Mr Obama is expected to announce the extra commitment on Friday when he outlines his new Afghanistan strategy.

Mr Levin was speaking outside the Senate after senators were briefed about Mr Obama's new strategy.
 

"There is a significant additional training group that will be going I think in June," he told reporters.

Asked if the number of American trainers was 4,000, he replied: "I won't disagree with that."

Mr Obama has also briefed Afghan President Hamid Karzai about the new strategy by telephone.
 

The Afghan leader has been defending himself against allegations that corruption is widespread within his government and is fuelling the country's instability.

Mr Karzai said on Thursday that such charges were exaggerated and politically motivated.

 

'New idea'
 

Mr Obama said earlier in the week that he was seeking "an exit strategy".

"There's got to be a sense that this is not a perpetual drift," he told US TV channel CBS.
 

The broad outlines of Mr Obama's new strategy are already clear, the BBC's Adam Brookes reports.

The war will continue for some time to come and the US will encourage Nato allies to step up their military effort too while non-Nato countries will be asked to at least not oppose it.
 

Washington will also try to draw Afghanistan policy and Pakistan policy into a coherent whole, our correspondent says.

Eventually, the US hopes Afghan security forces will take over security operations and allow foreign forces to draw down.
 

Our correspondent adds that Mr Obama may also seek, after seven years of operations, to explain America's goals.

As the "Global War on Terror" fades from our lexicon, he may set out a unifying idea to replace it, Adam Brookes says.