US Secretary of State Condoleezza Rice made the remarks on Thursday after two newspapers reported that the Bush administration had word of a "Syrian hit list" targeting Lebanese leaders.
"What we don't want is that there is a pattern now of assassination of key figures, because that would be very, very destabilising in Lebanon, and I think it would have to point a finger at those forces that have been destabilising in Lebanon," Rice said in an interview with Charlie Rose on PBS.
Druze leader Walid Jumblatt also went on television late on Thursday, saying: "I believe the entire opposition is being targeted," repeating an accusation he has often made since the murder in February of former premier Rafiq al-Hariri.
"The assassinations will continue with or without the knowledge of Syrian President Bashar al-Assad."
Jumblatt was speaking only a week after the latest political killing - of prominent anti-Syrian journalist Samir Kassir.
Lebanon's pro-Syrian government and its political masters in Damascus have denied widespread allegations that they were behind the two killings, as well as a series of bombings since al-Hariri's death that have killed three others.
Possible hit list
But Bush administration officials say the United States has received credible word that Syria has developed a hit list targeting prominent Lebanese political leaders, The New York Times and The Washington Post reported on their websites.
Both newspapers cited a senior administration official as saying Syria's plan was aimed at creating instability in an attempt to regain control of Lebanon.
"When Lebanese sources tell us they are hearing that the Kassir killing will be followed by others, we take it seriously"
Senior US official
In its Friday edition, The New York Times said Washington had credible information that Syrian operatives planned to assassinate senior Lebanese politicians and that Syrian military intelligence forces were returning to Lebanon to create an environment of intimidation.
The official said the information had come from a variety of Lebanese sources. "We assess it as credible," the source said.
"This is a moment when many politicians are facing overt Syrian intimidation in the middle of the election period," said the official. "When Lebanese sources tell us they are hearing that the Kassir killing will be followed by others, we take it seriously."
But intelligence officials said they could not immediately substantiate the reliability of the information cited by the administration official, The New York Times reported.
There was no immediate comment on the reports from the US State Department.
"We need to be very clear that we expect a full investigation of the assassination of Mr Kassir, following the assassination of Mr Hariri," Rice said in the interview.
Asked whether Syrian intelligence agents had withdrawn from Lebanon, she said: "I think some of us have our doubts and we need to keep pressure on the Syrians to be transparent about what they are doing in Lebanon."
A full investigation has been
called for into Kassir's murder
Rice added that the Bush administration was "being very clear to people that we want an international spotlight on what is going on in Lebanon" so the Lebanese can hold elections free of foreign influence.
A UN Security Council resolution in September demanded Syria withdraw all its forces from Lebanon, which were deployed there for 29 years. Damascus did so last month.