The Bank of Japan said it would do its "utmost" to provide market liquidity and ensure the stability of financial markets on Friday after the quake struck.
The BoJ also announced that the two-day policy board meeting previously scheduled for Monday and Tuesday would now be cut short and conclude on Monday, seen as a sign it may quickly implement extraordinary measures.
Japan's government said Sunday it expects a "considerable" economic impact from a huge earthquake and devastating tsunami that plunged the nation into what the prime minister called its worst crisis since the Second World War.
Economists say it is still too early to assess the cost of the destruction from the record 8.9-magnitude quake and the 10-metre wall of water that laid waste to swathes of the northeastern coast and triggered an atomic emergency.
The official death toll so far is 1,200, but is certain to rise substantially, with one hard-hit prefecture saying as many as 10,000 could be dead.