President Bush told reporters at the White House he is concerned about news reports saying Syria is still trying to influence political developments in Lebanon.
"I have been disturbed by reports I read in today's newspaper that said that Syrian intelligence officers might still be in Lebanon, might still be there," said Mr. Bush.
Syria says all of its military and intelligence officials left Lebanon last April. The withdrawal followed intense international pressure after the assassination of former Lebanese Prime Minister Rafik Hariri.
Syria had stationed troops in Lebanon since that country's civil war in the 1970s.
However, administration officials say senior Syrian intelligence personnel have been seen back in Lebanon, and President Bush says that is not acceptable.
"Our message to Syria, and it is not just the message of the United States, the United Nations has said the same thing, is that in order for Lebanon to be free is for Syria to not only remove her military, but to remove intelligence officers as well. Obviously we are going to follow-up on these troubling reports and we expect the Syrian government to follow-up on these troubling reports," Mr. Bush added.
White House spokesman Scott McClellan called on the United Nations to send verification teams back to Lebanon.
U.N. Secretary-General Kofi Annan says he is considering sending such a group to check reports that Syrian intelligence officials may still be operating in the country.
A previous U.N. verification team confirmed that all Syrian soldiers had been removed, but said it could not be sure all intelligence agents had left the country.
The White House spokesman says Syria's long presence in the country has established a lingering environment of intimidation.
Mr. McClellan's remarks follow U.S. news reports that Syrian operatives are planning to assassinate senior Lebanese political leaders.
An administration official who asked not to be identified says there is a Syrian hit list and many Lebanese politicians are facing intimidation during staggered elections that continue this month.