Dana Perino, White House spokeswoman, said the house speaker "should take a step back and think about the message that it sends.
"This is a country that is a state sponsor of terror, one that is trying to disrupt the Siniora government in Lebanon and one that is allowing foreign fighters to flow into Iraq from its borders."
Perino said Bashar al-Assad, the Syrian president, "probably really wants people to come, and have a photo opportunity, and have tea with him, and have discussions about where they are coming from. But we just think it's a really bad idea".
Pelosi arrived in Israel on Friday in what is her second fact-finding trip to the Middle East since taking over as speaker in January.
The group planned to meet Ehud Olmert, the Israeli prime minister, and to travel to the West Bank to meet Mahmoud Abbas, the Palestinian president, according to Ellison's spokesman, Rick Jauert.
Others travelling with Pelosi include Keith Ellison, the first Muslim elected to congress, and Tom Lantos of California, chairman of the house foreign affairs committee.
Syria's embassy in Washington on Friday hailed the trip as "momentous" and expressed hopes it may change sorely strained relations with the US.
|A handful of US politicians met al-Assad|
in Damascus in December [EPA]
Ahmed Salkini, an embassy spokesman, said "this is definitely a momentous visit", but that "it all depends at the end on how it is going to affect the Bush administration's decision-making".
He said "anyone that wants to come and engage with Syria is more than welcome", noted that three members from Bush's Republican party were currently in Syria.
The three were visiting Syria on Friday and Saturday to discuss US-Syria relations and regional issues, a US embassy statement said.
The representatives were Frank Wolf, a Virginia Republican, Joe Pitts, a Pennsylvania Republican, and Robert Aderholt, an Alabama Republican.
In December, a handful of Republican and Democratic politicians visited Damascus and met al-Assad after the Iraq Study Group recommended a stepped-up diplomatic effort involving Syria and Iran to help calm the violence in Iraq.
The Bush administration has resisted that recommendation and condemned the politicians' visits.
Washington accuses Syria of permitting Muslim fighters to cross its border into Iraq, of supporting Palestinian fighters and of funnelling arms to the Lebanese Shia group Hezbollah, which fought a 34-day war with Israel last summer.
Syria has denied it allows fighters to cross over from its territory into Iraq and argues Iraq and the US have not done enough to police the border.
Relations had reached a low point in early 2005 when Washington withdrew its ambassador in protest against the assassination of Rafik al-Hariri, the former Lebanese prime minister, who was killed in a Beirut truck bombing that his supporters blamed on Syria.
Syria has denied involvement.
US officials held their first direct, high-level contact with Syrian representatives in years when they met this month with officials from several Middle East countries in Baghdad to discuss Iraq.