The State Department has formally designated the Lebanon-based militant group Fatah al-Islam as a terrorist organization.
Since May, the al-Qaida inspired group has been battling the Lebanese military in a Palestinian refugee camp in northern Lebanon.
The order effectively cuts off Fatah al-Islam from the US financial system and bans US citizens from assisting it. A State Department spokesman said the group may have few assets affected by the designation, but that the move will help tighten international scrutiny of its activities.
On Monday, the Lebanese military reported one soldier was killed in clashes with Fatah al-Islam.
Lebanese forces continued firing artillery at the militants holed up in Nahr al-Bared camp near Tripoli. The fighting has killed more than 200 people, including at least 136 troops since May.
Tens of thousands of residents of the camp have fled the violence.
On Sunday, a senior Fatah al-Islam commander made a conditional offer of surrender, but the army swiftly rejected it and demanded the rebels give themselves up.