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CRAWFORD, Texas, March 1 (Reuters) - Ratcheting up pressure on NATO allies, U.S. President George W. Bush said on Saturday he would use the alliance's summit in Bucharest next month to urge them to send more troops to Afghanistan.

Bush, at a joint news conference with Danish Prime Minister Anders Fogh Rasmussen at the president's Texas ranch, said he expected NATO members to bear a "heavy burden" in Afghanistan and that he would "encourage people to contribute more."

His remarks added new impetus to a U.S. push in recent months for NATO allies to take on a greater role combating a resurgent Taliban and their al Qaeda allies in the country.

Some senior U.S. officials have said the dispute over troop levels in Afghanistan risks splitting the alliance between members willing and unwilling to fight.

Bush stopped short of naming the countries he believes should deploy more troops and acknowledged "certain political constraints on certain countries."

"I am going to go to Bucharest with the notion that we're thankful for the contributions being made and encourage people to contribute more," he said. The NATO summit is scheduled for April 2-4.

Germany, under pressure from Washington, has recently softened a longstanding insistence that it cannot exceed a self-imposed limit of 3,500 troops in Afghanistan, and no longer excludes reinforcements this year.

France has also signaled a willingness to send more troops. (Reporting by David Alexander, writing by Matt Spetalnick; Editing by Eric Beech)

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