Syria's army on Tuesday trained its sights on the town of Yabrud, the last rebel stronghold in the strategic Qalamoun region near Lebanon's border, after a string of battlefield victories.
The town is believed to be where a group of nuns from the historic Christian hamlet of Maalula have been transferred, reportedly in the hands of jihadist rebels from Al-Nusra Front.
In Spain, meanwhile, El Mundo newspaper said Spanish journalists Javier Espinosa and Ricardo Garcia Vilanova have gone missing in northern Syria.
The pair are believed to have been kidnapped in September by the Islamic State of Iraq and the Levant, but El Mundo's director Pedro Ramirez said "we believe they are alive and we believe they are well".
ISIL has carried out public executions and committed other human rights violations in the areas under its control as it has sought to impose a harsh version of Islamic law.
The group's growing influence has alarmed Western nations backing the rebellion, and on Tuesday opposition leader Ahmad al-Jarba accused the regime of supplying it arms in order to divide the rebels and tarnish their cause.
The Islamic Front, the largest Islamist rebel alliance in Syria's 33-month conflict, meanwhile seized a Turkish border crossing from the Western-backed Free Syrian Army, said the Syrian Observatory for Human Rights.
The Islamist alliance, which rejected the FSA's authority last week, does not include ISIL or the Al-Nusra Front, another Al-Qaeda-linked group.
The Observatory said regime forces in Qalamoun were shelling Yabrud, a day after they captured the nearby town of Nabak.
Government forces, reportedly backed by fighters from Lebanon's Hezbollah movement and members of a pro-regime militia, have seized Nabak, Deir Attiya and Qara.
The Damascus-Homs highway, a key supply route from the capital to the centre of the country, is expected to reopen soon after being closed by the fighting in Qalamoun.
The regime advance along the highway allowed rebel fighters to re-enter Maalula last week, shortly after which a group of nuns from the Mar Takla convent were reported kidnapped. Religious officials have said the 12 nuns are in Yabrud.
The women appeared Friday in a video broadcast by the Al-Jazeera news channel, apparently in good health and denying they had been kidnapped.
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It was unclear who was filming the video, and whether the nuns were speaking under duress.
They were dressed in their traditional black robes and head coverings, but none were wearing their usual crucifixes.
Wife pleads for journalists' release
In Spain, El Mundo said journalists Espinosa and Vilanova had been kidnapped on September 16 in Raqa province.
Despite indirect contact with the kidnappers, which resulted in information last month suggesting the pair were still alive, there has been no progress towards their release, nor any demands made.
Espinosa's wife Monica Prieto, speaking at a news conference in Beirut, noted the journalists had braved fierce fighting to report on the plight of Syrians.
"Javier and Ricardo are not your enemy. Please, honour the revolution they protected, and set them free," she said.
The opposition Local Coordination Committees also said Tuesday that four prominent activists, including Razan Zeitouneh, who was honoured by the European parliament in 2011, had been kidnapped by members of the armed opposition.
In Geneva, meanwhile, the United Nations said it would begin sending 40 metric tonnes of aid from Iraq into Syria on Thursday.
Amin Awad, who heads the UN refugee agency (UNHCR) Syria response, told reporters in Geneva the aid would be flown from Arbil in Iraq's northern Kurdish region to Qamishli and Hasakeh in northeastern Syria.
UNHCR spokesman Peter Kessler told AFP it was "to my knowledge" the first such international airlift into Syria.
The Organisation for the Prohibition of Chemical Weapons, which received its Nobel Peace Prize Tuesday, said destruction of Syrian chemical weapons aboard a US ship could start by the end of January.
But OPCW director general Ahmet Uzumcu warned the timeline depended on security in Syria, adding "unfortunately the security situation has deteriorated over the past weeks."
An estimated 126,000 people have been killed in the conflict, which began as peaceful democracy protests in March 2011 but escalated into a civil war following a brutal crackdown by Assad's forces.