Secretary of State Condoleezza Rice (search) linked the assassination Tuesday of an anti-Syrian politician in Lebanon with the continued activities of longtime overlord Syria (search), but said she cannot be certain who is behind the killing.

"I do not know who was responsible for this and I don't want to say that I know who was responsible, because I don't," Rice said. "But there is a context and an atmosphere of instability. Syria's activities are a part of that context and that atmosphere and they need to knock it off."

Former Lebanese Communist Party leader George Hawi (search) was killed instantly when his car blew up in Beirut. It was the second killing of an anti-Syrian figure this month, and closely followed elections won by an anti-Syrian slate.

Rice said there is "uncertainty about Syrian activities in Lebanon," despite Syrian claims that it pulled the last of its troops and intelligence forces out of the country in April.

The Bush administration has cast public doubt on the Syrian claims, but Rice made the allegation specific. "Their visible forces are gone but they clearly are still acting ... in Lebanon," she said.

Syria held political and military sway in tiny neighboring Lebanon for some three decades. In addition to the armed troops on Beirut streets, Syrian intelligence forces were often a shadowy but pervasive force in Lebanese daily life.

Rice answered a question about the killing by alluding to claims from U.S. officials this month that Syria may be running down a hit list of opposition figures.

"You know that we have been concerned about the potential for further assassinations of political figures in Lebanon — anti-Syrian political figures," Rice told reporters after completing a four-day diplomatic tour of the Middle East.

Rice spoke on a flight to Brussels, where she will help lead an international meeting on Iraq's reconstruction. Syria has been invited to the conference, and the current, Syrian-allied government of Lebanon may also be there.

Later in the week Rice will be in London for a separate gathering of foreign ministers from the Group of Eight economic coalition. While there, Rice is expected to discuss developments in Lebanon with her French counterpart.

The United States and France jointly sponsored a United Nations resolution last year which demanded Syrian withdrawal from Lebanon. The Damascus government largely ignored the demand until this spring. Anti-Syrian resentment boiled over into street demonstrations in February after the killing of former premier Rafik Hariri (search), a prominent opponent of Syrian influence.