Lebanon has arrested a wife and son of Abu Bakr al-Baghdadi, the self-proclaimed caliph of the jihadist Islamic State (IS) group that has seized large chunks of Iraq and Syria, a military source said.

The two were detained by military intelligence 10 days ago near the town of Arsal, close to the border with Syria, the source told AFP, saying only that the woman was Syrian and that the boy was eight or nine years old.

But a security source named the woman as Saja al-Dulaimi, saying she belonged to the powerful Dulaimi tribe that straddles Syrian and Iraq, and that she was from the eastern Syrian city of Deir Ezzor.

However, a French expert on jihadist movements, Romain Caillet, has raised doubts about whether the woman arrested is Baghdadi's wife if she is indeed Saja al-Dulaimi.

He said Dulaimi had been exchanged for 13 Christian nuns and three helpers kidnapped in the Syrian town of Maalula in December by the Al-Nusra Front, the bitter enemy of IS, making it unlikely she would be Baghdadi's wife.

It is not known how many wives and children Baghdadi has, but Islamic law allows men to marry four women and IS has encouraged its members to wed more than once.

Lebanese daily As-Safir, which first reported the arrest, said it was carried out "in coordination with foreign intelligence agencies". The woman was travelling on a false passport, it added, without elaborating.

The security source said the pair were taken to the defence ministry headquarters in Yarze, just outside Beirut, "where investigations were continuing".

The arrest had been kept secret while security arrangements were made.

- Soldiers killed in ambush -

Separately, the National News Agency said the military had arrested Tuesday the wife and brother-in-law of Nusra military commander Anas Sharkas, known as Abu Ali al-Chichani, in the northern town of Zgharta.

Under Baghdadi's leadership, IS has become arguably the most brutal, powerful and wealthiest jihadist organisation in the world.

He revived the fortunes of Iraq's struggling Al-Qaeda affiliate and turned it into the independent IS group, which has seized large parts of Iraq and Syria, carrying out atrocities in both countries.

Baghdadi was put on the US "terrorism" watch list in October 2011, and there is a $10 million (eight million euro) bounty for his capture.

In June he was declared a "caliph" in an attempt to revive a system of rule that ended nearly 100 years ago with the fall of the Ottomans.

US officials say he was born in 1971 in Iraq, and joined the insurgency that erupted shortly after the 2003 US-led invasion.

He spent time in an American military prison in the country, and is believed to have taken the reins of IS's predecessor, the Islamic State of Iraq (ISI), in 2010, after two of its chiefs were killed in a raid.

The alleged capture of his wife and son comes as Lebanon struggles to secure the release of 27 Lebanese soldiers and policemen held hostage by IS and Nusra, Al-Qaeda's franchise in Syria.

The security force members were captured when the jihadists briefly overran Arsal in August, sparking fierce battles with Lebanese troops.

Three have been executed so far, and the jihadists have threatened to kill the remaining hostages unless there is a deal to free Islamist prisoners in Lebanon.

Qatar has reportedly sought to mediate negotiations for their release but without success so far.

In other developments Tuesday, gunmen killed six Lebanese soldiers and wounded one in an ambush on their patrol in the east of the country near the border with Syria, the military said.

The attack, in the desert region of Ras Baalbek, was followed by clashes between the army and those behind the ambush, the source added.

Lebanon has tried to insulate itself from the war that has engulfed neighbouring Syria, as well as the threat posed by jihadist groups like IS and Al-Nusra.

But the conflict has regularly spilled over into the tiny Mediterranean country, which is hosting more than one million Syrian refugees.