By Sue Pleming

WASHINGTON, March 27 (Reuters) - Washington estimates up to 90 percent of suicide bombers in Iraq enter the country via Syria which has not acted to stop this flow of attackers, the U.S. State Department's Iraq adviser said on Tuesday.

David Satterfield, who is Secretary of State Condoleezza Rice's key adviser on Iraq, said Syria had an obligation to stop "jihadists" crossing into Iraq where suicide bombings are an almost daily occurrence.

He estimated 90 percent of suicide bombers in Iraq were foreigners and while the mix of nationalities changed, some 85 to 90 percent of them crossed over from Syria.

"They (suicide bombers) see Syria as a more accommodating country through which to transit across the border to come into Iraq to perpetrate their terror," Satterfield said in a speech to the Washington Institute for Near East Policy, .

"It has to stop, it is not in Syria's long term interests to let this violence continue," he added.

Syria has denied it allows insurgents to cross over from their territory into Iraq and says it is Iraq and the United States that have not done enough to police the border.

Asked whether Iraqi and U.S. security forces were playing their part in curbing the flow of attackers, Satterfield replied: "We and the Iraqi security forces have done our best. It is a long, long border."

Satterfield attended a March 10 meeting of Iraq's neighbors in Baghdad which included both Syria and Iran. The U.S. delegation made clear their displeasure with both Syria and Iran for their "interference" in Iraq.

"We would hope that the Syrian government understands as well that its rhetoric for a peaceful and stable Iraq has to be matched by actions," said Satterfield.

Washington, which is also at loggerheads with Iran over its nuclear program, has repeatedly accused Tehran of meddling in Iraq and of supplying explosives used against U.S. troops. Tehran denies any role in supplying arms.

"We would hope that Iran understands that they cannot continue those activities unchallenged and if there is a belief on behalf of the government in Tehran that the United States will not respond, then they are wrong," said Satterfield, without specifying what this response would be.

As a follow-on from the March 10 meeting, Rice is expected to join other foreign ministers from Iraq's neighbors early next month to discuss the violence in Iraq and how to contain it.

Satterfield said a date and location had not yet been set for the ministerial meeting but he expected it to take place in the next few weeks.

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