"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."