BEIRUT (Reuters) - The Lebanese army detained a wife and daughter of Islamic State leader Abu Bakr al-Baghdadi as they crossed from Syria nine days ago, security officials said on Tuesday, in a move seen as likely to put pressure on the Islamist chief.

The woman was identified as Saja al-Dulaimi, an Iraqi, by a Lebanese security official and a senior political source.

The Lebanese newspaper As-Safir reported she had been detained in coordination with “foreign intelligence”.

A Lebanese security source said the arrest was “a powerful card to apply pressure” in negotiations to secure the release of 27 members of the Lebanese security forces captured by Islamic militants - a view shared by other Lebanese officials who confirmed the arrest.

However, U.S. Secretary of State John Kerry dismissed any suggestion that Washington might also try to use similar tactics to free prisoners. “We do not engage in that type of negotiation. Period,” he told a news conference in Brussels.

A senior Lebanese security official said Baghdadi’s wife had been traveling with one of their daughters, contradicting earlier reports that it was his son. DNA tests were conducted to verify it was Baghdadi’s child, the official said.

They were detained in northern Lebanon after Baghdadi’s wife was found with a fake passport, officials said. Investigators were questioning her at the Lebanese defense ministry.

There was no immediate reaction from Islamic State websites, although some supporters rejected the report.

Fawaz Gerges, a Middle East expert at the London School of Economics, said the arrest indicated that the American-led coalition seemed to have solid intelligence in Syria and Iraq.

“I talked to a few people who told me this was a coordinated arrest between American intelligence services and the Lebanese army,” he told Reuters.

“If I were Abu Bakr Baghdadi, I would be very anxious that they are getting very close,” he said. “This is a development ... that is very alarming for ISIS, in particular the top leadership.”

A man purported to be the reclusive leader of the militant Islamic State Abu Bakr al-Baghdadi has made what would be his first public appearance at a mosque in the centre of Iraq's second city, Mosul, according to a video recording posted on the Internet on July 5, 2014, in this still image taken from video. REUTERS/Social Media Website via Reuters TV

A U.S. government source said Washington was not sure how recently the woman had been with Baghdadi, and how much useful information, if any, she might have.

The U.S. embassy in Lebanon said: “This was an operation by the government of Lebanon.” It had no further comment.

Dulaimi was one of 150 women released from a Syrian government jail in March as part of a prisoner swap that led to the release of 13 nuns taken captive by al Qaeda-linked militants in Syria, according to media reports at the time.

A source with contacts with Iraqi intelligence said the captured woman was an Iraqi wife of Baghdadi’s, but could not confirm her name. There was cooperation between Iraqi and Lebanese authorities leading up to her capture, the source said.

Baghdadi has three wives, two Iraqis and one Syrian, according to tribal sources in Iraq.

CALIPHATE

Islamic State has declared a caliphate and seized wide areas of Iraq and Syria, Lebanon’s neighbor to the east.

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

They have arrested dozens of Islamic militants suspected of staging attacks to expand Islamic State influence in Lebanon.

On Tuesday, at least six Lebanese soldiers were killed by gunmen from Syria who attacked an army patrol near the border.

A U.S.-led alliance is seeking to roll back Islamic State’s gains in Iraq and Syria, where the group is seeking to reshape the Middle East according to its radical vision of Islam.

An Islamic State fighter denied Baghdadi’s wife had been arrested. “I have checked with our leaders and they said it was false news,” he said from inside Syria.

Spillover from the Syrian conflict has repeatedly jolted neighboring Lebanon. Militants affiliated to the al Qaeda-linked Nusra Front and Islamic State are demanding the release of Islamists held by the Lebanese authorities in exchange for the captured members of the Lebanese security forces.

The United States is offering $10 million for information on Baghdadi, an Iraqi, whose real name is Ibrahim Awwad Ibrahim Ali al-Badri al-Samarai.

Baghdadi called for attacks against the rulers of Saudi Arabia in a speech purported to be in his name last month.

A CV of Baghdadi published on social media in July by Islamic State sympathizers described him as married but gave no further details.

Born in 1971, Baghdadi comes from a family of preachers and teachers, according to a biography on Islamist forums that says he studied at the Islamic University in Baghdad.

According to U.S. media reports, Baghdadi had been detained at Camp Bucca, a U.S.-run prison in Iraq, before becoming head of the Islamic State of Iraq in 2010, a predecessor to Islamic State, which expanded into Syria in 2013.

In June this year, his group named him “caliph for the Muslims everywhere”. Although he is rarely pictured, a video released in July showed him preaching in a mosque in Mosul.