Syria claims all 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," said one senior US State Department official on Thursday in London on condition of anonymity.
The United Nations plans to send a verification team back to Lebanon to investigate whether intelligence forces remain, but the US official, who requested anonymity because sensitive intelligence is involved, said the answer is already in.
"We're certain," the official said.
Until now, the US has only strongly suggested the continued presence of Syrian intelligence agents, a claim made openly by opposition politicians in Lebanon.
Rice met French Foreign Minister
Douste-Blazy (R) in London
A second US official said US Secretary of State Condoleezza Rice 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 UN resolution demanding full withdrawal.
The US and France sponsored that resolution last autumn, and Lebanon was a main topic of a meeting on 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 three 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that have been going on in Lebanon, and also about the need to Syria to make certain that all of its forces are withdrawn," Rice said after her meeting.
"We expressed concerns about the assassinations that have been going on in Lebanon"
US Secretary of State
Politicians opposed to the current Syrian-allied government in Damascus won the election and will soon try to form a government.
The harsher US allegations appear timed to draw international attention to Lebanon at a moment when Syrian influence could wither or regenerate.
None of the allegations are specific and the Bush administration has offered no public evidence for them.
Heat on Syria
Aljazeera's correspondent in Paris, Ayash Darraji, says the latest Western pronouncements need to be viewed against the backdrop of developments in Lebanon.
Observers say the statements are related to the assassination of the anti-Syrian Lebanese communist politician, George Hawi, and the recent visit of Saad al-Hariri to Paris.
Continued killings in Lebanon are
ratcheting up pressure on Syria
It may be recalled that the French Foreign Ministry and the French authorities in general praised the recently conducted elections in Lebanon without commenting on its outcome.
Darraji says the renewal of the diplomatic offensive against Damascus suggests that France and the US may have in their possession evidence of continued Syrian intelligence presence in Lebanon, although Paris has not made any declaration in this regard.
In any event, the harmony between the US and French pronouncements on this issue is indicative of close coordination between the two countries with regard to Syria.