The killing of Lebanese politician Pierre Gemayel may be the "first shot" in a coup against the government, a top US official has said.
Businesses remain closed in Lebanon in protest at the killing
John Bolton, the American ambassador to the United Nations, said recent probes into political killings in Lebanon suggested Syrian involvement.
He told the BBC that if Syria was deemed to have been involved, the implications were serious.
Saturday sees the second day of a strike protesting at the killing.
Many shops closed on Friday with business leaders saying they hoped the action would move Lebanon closer to a "national dialogue" and ward off the threat of division and street protests.
Mr Gemayel, a Maronite Christian who was the industry minister, was shot in his car in a Christian area of Beirut on Tuesday.
Many Lebanese accuse Damascus of orchestrating the 34-year-old's murder, although Syria explicitly denies any role.
Speaking in a BBC interview, Mr Bolton said Mr Gemayel's killing might be part of a wider plot.
"A few weeks ago the White House took the unprecedented step of saying that Syria and Iran, acting through Hezbollah, were on the verge of staging a coup d'etat against the democratically elected government of Lebanon, and I have to say that this assassination of Pierre Gemayel might well be the first shot in that coup."
Many Lebanese are still mourning Gemayel
He said did not want to pre-judge any investigation into Mr Gemayel's death, but proof of Syrian involvement would show it was "not just a supporter of terrorism but is a state actor in a terrorist fashion".
"I think the United States has to take that into account when it decides whether and to what extent to deal with a country like that.
The BBC's Nick Miles in Washington says the US is in a diplomatic quandary.
It seems increasingly likely that a key advisory panel on future strategy in Iraq will suggest bringing in Syria to create a long-term solution to the violence, he says.
But our correspondent adds the assassination of Mr Gemayel has made that all the more difficult.
The Lebanese cabinet is due to meet on Saturday to approve a United Nations plan for an international tribunal to try those accused of murdering the former Prime Minister, Rafiq Hariri.
Syria has been implicated in the bombing that killed Mr Hariri last year, but denies involvement.
Correspondents say the Lebanese cabinet is expected to give its final approval to the tribunal, and this is likely to increase tensions with pro-Syrian politicians.
Earlier this month six pro-Syrian ministers resigned from the cabinet, saying they wanted a greater role in government.
The death or resignation of two more cabinet ministers would bring down the government.
Meanwhile, the UN Security Council has agreed to a request from the Lebanese government to help investigate Mr Gemayel's murder.
His killing will be probed by the same tribunal that implicated senior Syrian officials and pro-Syrian Lebanese in the killing of Mr Hariri.
Correspondents say the government is accusing Hezbollah of exploiting the delicate political situation in Lebanon to try and block the UN-backed probe into the Hariri killing.
Pro-Syrian groups have already said the UN plan is illegal under Lebanon's constitution.
In 2005, Syria withdrew its troops from Lebanon after a presence of 29 years, following massive domestic and international pressure following the assassination of Mr Hariri.