The United States is certain that Syrian military intelligence operatives remain in Lebanon (search) in defiance of international demands to withdraw all forces and agents, two senior U.S. officials said Thursday.

Syria (search) claims all its forces quit Lebanon in April after some three decades as the dominant political and military force there.

"There is no question that Syrian military intelligence agents have stayed behind, and that they are exerting a highly negative influence on the situation," one senior State Department official said on condition of anonymity.

The United Nations (search) plans to send a verification team back to Lebanon to investigate whether intelligence forces remain, but the U.S. official, who requested anonymity because sensitive intelligence is involved, said the answer is already in.

"We're certain," the official said.

Until now the United States has only strongly suggested the continued presence of Syrian intelligence agents, a claim made openly by opposition politicians in Lebanon.

A second U.S. official said Secretary of State Condoleezza Rice (search) heard no quarrel on the Syria assertion among European diplomats meeting in London on Wednesday and Thursday.

Neither official offered specific evidence for the claim, an estimate of the number of Syrians who may be in Lebanon or an accounting of the alleged agents' activities.

Syria says it has complied with a U.N. resolution demanding full withdrawal. The United States and France sponsored that resolution last fall, and Lebanon was a main topic of a meeting Thursday between Rice and her French counterpart, Foreign Minister Philippe Douste-Blazy.

As she has done before, Rice hinted that holdover Syrian agents were behind recent political assassinations in Lebanon but offered no specifics.

"We expressed concerns about the assassinations that have been going on in Lebanon, and also about the need to Syria to make certain that all of its forces are withdrawn," Rice said after her meeting.

Rice is in London to help prepare for next month's Group of Eight (search) economic summit in Scotland, which President Bush plans to attend. The day's meetings complete a week of hopscotch foreign travel for Rice in the Middle East and Europe.

The Bush administration has turned up the heat on Syria since completion of national elections in Lebanon last weekend.

Politicians opposed to the current Syrian-allied government in Damascus won the election and will soon try to form a government. The harsher U.S. allegations appear timed to draw international attention to Lebanon at a moment when Syrian influence could either wither or regenerate.

None of the allegations are specific and the Bush administration has offered no public evidence for them.

On Wednesday, Rice accused Syria of exporting terrorism over its border with Iraq. A day earlier she linked Syria to the latest assassination of an anti-Syrian politician but said she cannot be certain who is behind the killing.

"I do not know who was responsible for this and I don't want to say that I know who was responsible, because I don't," Rice said then. "But there is a context and an atmosphere of instability. Syria's activities are a part of that context and that atmosphere and they need to knock it off."

Former Lebanese Communist Party leader George Hawi (search) was the second anti-Syrian figure killed this month and the third this year.

"These are not random killings," White House press secretary Scott McClellan said then. "These are targeted assassinations of political figures. It is clearly an attempt to intimidate the people of Lebanon and to undermine progress toward a free and democratic future."

Britain's Foreign Secretary Jack Straw condemned the assassination of Hawi. "Since we know that Syria continues to exercise a great deal of influence within Lebanon we look to the Syrian government to do all that it can to ensure that those who are committing these outrageous assassinations stop and finish," he said during a break in talks with his G-8 colleagues.

Syria held political and military sway in tiny neighboring Lebanon for some three decades. In addition to the armed troops on Beirut streets, Syrian intelligence forces were often a shadowy but pervasive force in Lebanese daily life.