US Treasury Secretary Timothy Geithner says the government has begun borrowing from the federal employee pension fund to keep operating without surpassing its debt limit.

Geithner says in a letter to congressional leaders that the move will free up $US156 billion in borrowing authority while Congress debates increasing the $16.4 trillion debt limit.

The government reached its borrowing limit on December 31, but began using bookkeeping manoeuvres to keep from surpassing it.

Geithner has told congressional leaders that Treasury expects to exhaust those measures by mid-February to early March.

The latest action has been taken by other Treasury secretaries and will not put in jeopardy any monthly pension payments. Geithner said he will replace the funds removed from the pension account after the borrowing limit is raised.