The United States is considering new sanctions against Russia, proposing to cut some of its top banks from dollar transactions and reducing their ability to service dollar-denominated obligations, sources told Reuters.

Sanctions would come into a force if Russia invaded Ukraine, a move the Kremlin denies is on the cards despite a large Russian troop buildup near its neighbour's borders.

Banks' liquid forex assets, such as cash held by lenders directly and in the central bank as well as short-term loans and other non-rouble tools which could be easily claimed, stood at $45 billion in December, the central bank said in a report.

Earlier on Monday, Russian ratings agency ACRA estimated that the country's banks imported $5 billion worth of forex banknotes in December, double from a year ago, in a pre-emptive step in case sanctions create increased demand.

The central bank declined to comment on ACRA's calculations, nor did it provide a breakdown for the banks' liquid forex assets disclosed in its report Monday.

However, it said that last month, lenders had 21% of their clients forex funds and 15% of banks' liabilities covered by liquidity - a protection cushion if customers demand a speedy withdrawal. That was up from 18% and 13% in December, respectively.

Although retail clients withdrew 810 billion roubles from their current accounts last month, Russian banks managed to post 164 billion roubles ($2 billion) in January profit, up by 47% from the previous month, the central bank said.

($1 = 79.2050 roubles)

(Reporting by Elena Fabrichnaya; Writing by Katya Golubkova; Editing by Alexander Smith and Frank Jack Daniel)

By Elena Fabrichnaya and Katya Golubkova