Lebanese soldiers in Islamic State captivity since 2014 are almost certainly dead, a senior security official said on Sunday, just hours after the army announced a ceasefire to hold talks over their fate.

The ceasefire halted the fighting in an Islamic State enclave at the Syria-Lebanon border, where the militants have been fighting the Lebanese army on one front and Hezbollah with Syrian troops on the other.

Islamic State has held nine Lebanese soldiers captive since 2014, when it briefly overran the northeast border town of Arsal with other militants - one of the worst spillovers of the Syrian conflict. The fate of the troops had been unknown since then.

The Lebanese army announced its ceasefire took effect at 7 a.m (0400 GMT). Hezbollah and the Syrian army also declared a ceasefire in their own attack against Islamic State in Syria's western Qalamoun region, Hezbollah's al-Manar TV said.

The ceasefire held on both sides of the border throughout the day, as sources said plans for an evacuation of the remaining militants were under discussion.

The fighting began a week ago when the Lebanese army, and Hezbollah together with Syrian government forces, launched separate but simultaneous assaults.

Both offensives have advanced towards the Syria-Lebanon frontier from opposite sides, hemming the militants into a small zone in the arid hills straddling the border. Lebanon's army and Hezbollah have each said the battle was nearing victory.

The Islamic State pocket marks the last militant foothold along the Syrian-Lebanese frontier.

Defeating Islamic State there would end years of insurgents from Syria's six-year war holding territory in the mountainous border region, and allow the two countries to consolidate control of the frontier.

The head of Lebanon's internal security agency said the army and security forces had retrieved remains thought to belong six of the soldiers and were conducting digs on Lebanese land for two others. DNA tests were needed to confirm the identities.

"We believe, almost certainly, that these are the remains of the soldiers," said the general, Abbas Ibrahim, who mediated talks between the army and the militants. The whereabouts of the ninth soldier remain unknown.

The soldiers' families had gathered in central Beirut on Sunday, saying they would hold out hope until the last minute.

Ibrahim arrived in the afternoon to give them the news.

"I know this is a difficult moment ... Liberating the land calls for offering our souls to this country," he said.

"We do not bargain. We are in the position of the victor and are imposing conditions."

A military source had said earlier that Islamic State fighters had "succumbed ... and asked for the negotiations".

EVACUATION TALKS

Hezbollah has played a major role in fighting militants along the border during Syria's war, and has sent thousands of fighters to support Syrian President Bashar al-Assad.

Earlier this month, Nusra Front militants and a Syrian rebel group withdrew from Lebanon's northeastern border region. They departed for insurgent territory in Syria after Hezbollah routed them in offensives with the Syrian army.

In a speech last week, Hezbollah leader Sayyed Hassan Nasrallah said his Iran-backed group had begun talks with Islamic State on a truce.

The Lebanese army has said it is not coordinating its attack with the Syrian army or Shi'ite Hezbollah, which Washington classifies as a terrorist group.

Any joint operation between the Lebanese army and either the Syrian army or Hezbollah would be politically sensitive in Lebanon and could jeopardise the sizeable U.S. military aid the country receives.

Hezbollah and its allies have been pressing the Lebanese state to normalise relations with Damascus, testing Lebanon's official policy of neutrality towards the conflict next door.

Calls for closer ties come as Assad's government has put rebels on the back foot and shored up its rule over the main urban centres in Western Syria.

A Western diplomat praised the Lebanese army's performance in the border battle in "a risky and complex operation" and said it would have been "simply unimaginable" a decade ago.

"We see no evidence of substantive cooperation (between the army and Hezbollah)," the diplomat added.

A source familiar with the talks said Hezbollah and the Lebanese army had been communicating over the ceasefire and negotiations.

The Islamic State fighters had asked Hezbollah and the Syrian army to let them withdraw to Syria's eastern province of Deir al-Zor, a pro-Damascus official had said.

Several hundred militants are still holed up in the enclave. If the deal continues smoothly, they would evacuate likely there, Ibrahim said.

Damascus has approved an Islamic State-Hezbollah deal that allows for transferring the militants into eastern Syria, state media said on Sunday.

As Islamic State has lost vast territories in Iraq and Syria, many of its forces have retreated to Deir al-Zor province, which remains almost entirely under its control.