Military supplies recently sent to Lebanon by the US included nerve gas and cluster bombs, claimed the militant Fatah Islam group, which is still staging fierce clashes against Lebanese troops, from their stronghold in the Nahr al-Bared Palestinian refugee camp.
"If they use unconventional weapons against us, we will respond with unconventional attacks everywhere," said a statement, read by the group's spokesman Abu Salim Taha. A Lebanese military spokesman said he had no reaction to "these false allegations which are not worth commenting on".
Earlier Saturday, three US transport planes carrying military aid to Lebanon's army arrived in Beirut, part of an international airlift to support the Lebanese troops in their fight against the Islamist group. Six planes carrying similar military aid from the US and Arab allies arrived on Friday.
Later, the leader of Fatah al-Islam issued a new threat in a videotaped message carried on Al Jazeera television. The group would fight "the Jews, the Americans and their loyalists", said Shaker al-Abssi, referring to Lebanese leaders.
Shortly later, clashes erupted between Lebanese soldiers and militants around the refugee camp in Lebanon's northern city of Tripoli. The clashes with heavy machineguns, grenades, mortars and artillery continued sporadically throughout the night.
It is unclear how this will affect a four-day-old truce that has mostly held up in recent days, despite sporadic gunfire.
Fatah al-Islam is led by a Palestinian but has little support among Lebanon's Palestinian refugee community of around 400,000. Thousands have fled the area in recent days.
Officials said they were giving mediators a chance to persuade the militants to surrender before ordering the army to move into the camp. The Lebanese army is banned from entering Lebanon's 12 refugee camps under a 1969 Arab agreement.