The mother of Mohammed Suleiman, one of at least six Lebanese soldiers who was killed during an ambush by gunmen in Ras Baalbek, carries her son's military clothes during his funeral in Jabal Mohsen, Tripoli December 3, 2014.

Lebanese military soldiers man a checkpoint, near the site of an ambush where gunmen killed at least six Lebanese soldiers, in the mountainous border town of Ras Baalbek December 3, 2014

Friends and family members carry the coffin of Mohammed Suleiman, one of at least six Lebanese soldiers who was killed during an ambush by gunmen in Ras Baalbek, during his funeral in Jabal Mohsen, Tripoli December 3, 2014

BEIRUT (Reuters) - Gunmen killed at least six Lebanese soldiers when they attacked an army patrol near the border with Syria on Tuesday, the army and a Lebanese security source said.

The source said the gunmen crossed from Syria to the mountainous border town of Ras Baalbek after dark and ambushed the patrol. Clashes between gunmen and a special army unit erupted after the attack and the army was able to retrieve the bodies of six of its soldiers.

"Six bodies just arrived at the local hospital of the town of Ras Baalbek," he told Reuters.

The army said in a statement that a "terrorist" group had attacked a patrol on a surveillance mission and killed six of the soldiers. The army had sent immediate reinforcement to the area and "took appropriate field procedures".

It was not immediately clear who carried out the attack which occurred on the day Lebanese security sources said that a wife and one of the daughters of Abu Bakr al-Baghdadi, leader of the ultra-hardline group Islamic State, were detained in the country as they crossed from Syria.

Lebanese security forces have cracked down on the group's sympathizers and the intelligence services have been particularly vigilant on the borders with Syria.

Spillover from the Syrian conflict has repeatedly jolted neighboring Lebanon. Militants from al Qaeda-linked Nusra Front and Islamic State attacked and briefly seized in August the Lebanese border town of Arsal, which is also close to Ras Baalbek, and took a group of soldiers prisoner when they pulled out.

The militants are demanding the release of Islamists held by the Lebanese authorities in exchange for the captured members of the Lebanese security forces.

(Reporting by Mariam Karouny; editing by Ralph Boulton)