Also Friday, Syrian soldiers surrounded an industrial area near Damascus after an al-Qaida linked rebel faction infiltrated it earlier this week, reportedly killing dozens of civilians, the government and activists said.
Jabhat al-Nusra, also known as the Nusra Front, entered buildings housing workers and their families, shooting men, women and children in Adra, northeast of the capital, according to the reports. Most residents of the area are from the minority Alawite and Druse sects, which largely support President Bashar Assad in his fight against mainly Sunni rebels.
The exact death toll could not be determined. State-run Syrian TV reported that scores of civilians have been killed since Wednesday, prompting the army to surround the town.
The Britain-based Syrian Observatory for Human Rights said it has documented the names of 19 civilians killed mostly Alawites and Druse and many more were feared dead after the rampage.
Akbik told reporters in London that it still wasn't completely clear how the Islamic Front came to control the warehouses at the Bab al-Hawa border crossing between Syria and Turkey.
"The reality is that the Islamic Front are holding, now, the hardware," he said. "They should return it."
Akbik said Gen. Salim Idris, the Free Syrian Army commander, was in southern Turkey meeting with members of the Islamic Front to demand the return of the weapons.
On Friday, the alliance's military spokesman, Islam Alloush, said he had no information about such a meeting, adding: "What is there to talk about?"
There are conflicting stories over how the Islamic Front won control of the warehouses. The Islamic Front denies that it took the weapons by force, saying it was responding to a call for help from the Free Syrian Army after the group came under attack from unknown gunmen.
Akbik refused to give any details on what happened, saying the situation was "foggy," but said ammunition mostly AK-47 bullets and heavy machine guns were seized.
U.S. Secretary of State John Kerry said U.S. officials have been in touch with Idris to inventory the equipment that was in the warehouse raided by the Islamic Front.
"We are in discussions with our friend and consulting with the opposition about our next steps in support of the Syrian people," he said at a press conference in Jerusalem.
With just about a month to go before the start of internationally brokered peace talks to end the civil war, Assad's forces have stepped up a punishing offensive against rebels in a mountainous region near the border with Lebanon.
The fractured opposition movement has been losing ground militarily as rebel factions turn their guns against each other.
Despite the incident, Akbik said the opposition was still hoping to include Islamic fighters as part of their delegation to the peace talks which are scheduled to start Jan. 22, which aim to end the fighting that activists say has killed over 120,000 people in three years.
He said the opposition was "keen to form a delegation that will be as wide as possible" and that the Islamic Front was welcome to send a representative to the talks as part of the opposition delegation.
The Syrian conflict began in March 2011 as a popular uprising against Assad that quickly deteriorated into a civil war.
The country's 23 million people belong to a patchwork of different religious groups, and the three-year conflict has taken on increasingly sectarian overtones in the past year, particularly as fighting brigades composed of al-Qaida loyalists gain influence.
Assad is an Alawite, an offshoot of Shiite Islam, as are many of his security forces. Other minorities in the country including Christians, Druse and Shiites have mostly sided with Assad or remained on the fence, fearing a takeover of the country by Islamic extremists.
Also Friday, fighters of the al-Qaida-linked group the Islamic State of Iraq and the Levant kidnapped 120 Kurdish civilians from a town in the northern Aleppo province near the border with Turkey, the Observatory said. It said at least six women were among those taken by the rebel faction. The fate of the abducted civilians is unknown, the Observatory said.
Meanwhile in Lebanon, snow fell on northern and eastern regions where tens of thousands of Syrian refugees are staying, many of them in flimsy plastic tents. Syrian refugees struggled to keep tents in place and were seen gathering sticks of wood from nearby fields to use them for heating. Families crammed into damp, muddy tents struggling to keep warm. In some cases, Syrian children came out of their tents to play with the snow.
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Vinograd reported from London. AP writer Bassem Mroue contributed from Beirut.