"We are deeply concerned about Syria's interference and intimidation inside Lebanon," said White House spokesman Scott McClellan.
President Bush's chief spokesman said that Syrian "intelligence operatives are interfering in Lebanon's internal affairs."
Syria has said that all of its military and intelligence officials left Lebanon by April 26.
But McClellan disputed that claim.
"We have called on the United Nations to send verification teams back to Lebanon," McClellan said.
U.N. Secretary-General Kofi Annan has said he was considering sending a U.N. team back to Lebanon to check reports that Syrian intelligence officials may still be operating in the country.
"It's important for the international community to send a clear message to Syria that it must stop meddling in Lebanon," McClellan said. "I think the world is watching Lebanon closely."
He said that Syria's long presence had established a lingering "environment of intimidation."
"Syria has not fully complied with the Security Council resolution and the demands of the international community" to withdraw all its forces from Lebanon, the spokesman added.
McClellan declined to discuss details of Syria's alleged continued involvement in Lebanon, saying he wouldn't comment on "intelligence matters."
A U.N. team monitoring Syria's compliance on May 23 said all locations formerly used by Syria's military intelligence apparatus were empty, and it concluded that "no Syrian military intelligence personnel remain in Lebanon in known locations or in military uniform."
But Lebanese opposition leader Walid Jumblatt asserted earlier this week that Syrian intelligence officials had been spotted in eastern Bekaa Valley and central Lebanon.
"Syria needs to comply fully with the United Nations Security Council Resolution 1559, and that means getting all their intelligence operatives out of Lebanon," McClellan said.