The banks will release $180bn to lift the amount of credit available.
US stocks opened higher, echoing gains in Europe. Lloyds TSB's takeover of UK lender HBOS also dispelled some of the gloom hanging over the financial world.
But markets are expected to stay volatile on fears that more firms could succumb to the financial crisis.
The main Dow Jones index was up 1.34%, or 142.2 points, at 10,751.8, in early exchanges in New York.
President George W. Bush said he was closely monitoring the situation on financial markets and the recent actions taken by the Federal Reserve and other regulators were "necessary and important".
"We will continue to act to strengthen and stabilize our financial markets and improve investor confidence," he said.