The State Department is expected to announce the designation against al Qaeda-inspired Fatah al-Islam, which is suspected of having links with Syria, on Monday.
The designation imposes financial and travel restrictions on the group and its members, officials said Saturday, speaking on condition of anonymity because the designation is not yet public.
The officials said Secretary of State Condoleezza Rice signed off on the decision to place the radical group on the international terror list on Friday. The sanctions took effect with her signature.
The U.S. designation of Fatah al-Islam will bring to 43 the number of groups on the blacklist, which already includes many of the world's most notorious terrorist organizations.
The action against the Lebanese group comes as the Bush administration is stepping up efforts to distance Lebanon from Syrian influence and sporadic fighting between Lebanese troops and Fatah Islam militants.
It comes as Washington steps up efforts to free Lebanon from Syrian influence and amid serious clashes between Lebanese troops and Fatah Islam militants at the Nahr el-Bared camp that have killed at least 136 people since they erupted in May.
There was no immediate comment from Lebanese officials. Fatah Islam militants, who spoke to journalists by mobile phone from inside the Nahr el-Bared camp in the early days of the fighting, can no longer be reached.
The fighting is the worst internal violence in Lebanon since its 1975-90 civil war and has dragged on despite the Lebanese army besieging the camp to uproot the group.
The army has refused to halt its offensive until the militants completely surrender, but the gunmen have vowed to fight to the death.
On Wednesday, Fatah Islam said in a statement posted to a Web site that its No. 2 commander, Abu Hureira, had been killed in the clashes and celebrated the "martyrdom of a noble a noble brother," vowing to avenge his death.
The whereabouts Fatah Islam leader, Shaker Youssef Absi, are unknown.
Fuad Saniora, Lebanon's Western-backed prime minister, has said there are connections between Syria and Fatah Islam, which was formed last year but grew to prominence with the fighting.
Syrian President Bashar al-Assad has denied the charge.
The United States has in recent months boosted its attempts to support Saniora and his government as they face a continuing political crisis with pro-Syrian elements, including Lebanese President Emile Lahoud, the head of state.
On Thursday, Rice held talks with Lebanon's most senior diplomat in Washington, a Saniora appointee whom Lahoud has refused to accredit as the country's official envoy to the United States.
Rice's meeting at the State Department with Antoine Chedid, whose formal title is Charge d'Affaires, effectively recognized him as the country's ambassador and was intended as a diplomatic slight to Lahoud, officials said.
Chedid's predecessor in Washington, a pro-Lahoud diplomat who carried the title of ambassador, left the United States in late July after the State Department waged a behind-the-scenes campaign to have him replaced.
On August 1, President Bush signed an executive order allowing the Treasury Department to block the assets of anyone deemed to be destabilizing efforts to promote Lebanese security and sovereignty, a move seen as targeting Lahoud and his supporters as well as Syrian officials.