LONDON – America is certain that Syrian military intelligence operatives remain in Lebanon in defiance of international demands to withdraw all forces and agents, two senior American officials said yesterday.

Syria claims all of its forces quit Lebanon in April after about 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.

Later, France’s foreign minister, Phillipe Douste-Blazy, told reporters: “We’re wondering about Syrian intelligence who may still be active in Lebanon. The international community will not accept a situation where attacks on civilian individuals continue.”

That was a reference to three political assassinations in Lebanon, but Mr. Douste-Blazy did not specifically pin the killings on Syria.

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

“We’re certain,” the official said.

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

A second American official said that Secretary of State Rice heard no quarrel on the Syria assertion among European diplomats meeting in London on Wednesday and yesterday.

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. America and France sponsored that resolution last fall, and Lebanon was a main topic of a meeting yesterday between Ms. Rice and her French counterpart.

Ms. Rice is in London to help prepare for next month’s Group of Eight economic summit in Scotland, which President Bush plans to attend. The day’s meetings complete a week of hopscotch foreign travel for Ms. 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 American allegations appear timed to draw international attention to Lebanon at a moment when Syrian influence could either wither or regenerate.

On Wednesday, Ms.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,” Ms. 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.”

A former Lebanese Communist Party leader, George Hawi, was the second anti-Syrian figure killed this month and the third this year.

Britain’s foreign secretary, Jack Straw, condemned the assassination. “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 has held political and military sway in tiny neighboring Lebanon for about 30 years. In addition to the armed troops on Beirut streets, Syrian intelligence forces were often a shadowy but pervasive force in Lebanese daily life.