Syria on Wednesday missed a deadline to hand over all the toxic materials it declared to the world's chemical weapons watchdog, putting the program several weeks behind schedule and jeopardizing a final June 30 deadline.
At the same time, opposition activists say the Syrian air force is attacking the country's biggest city, Aleppo, with barrel bombs, forcing many to flee. Turkey was turning away some of those refugees because camps were now full.
Under a deal reached in October between Russia and the United States, which helped avert a U.S.-led missile strike against the government of President Bashar al-Assad, Syria agreed to give up its entire stockpile of chemical weapons by February 5.
Russia said on Tuesday its ally Damascus would ship more chemicals soon, but Western diplomats said they saw no indications that further shipments were pending.
Syria has said it would submit a handover timetable to the Organisation for the Prohibition of Chemical Weapons (OPCW), which won the Nobel Peace Prize last year, but gave no indication of when that would happen.
There have been no shipments since January 27 and the latest deadline was missed, said OPCW spokesman Michael Luhan. "It's a status quo until we get this plan."
In Washington, White House spokesman Jay Carney said the United States was "absolutely not" concerned that the chemical weapons agreement was falling apart, but added that "Syria must abide by its commitments."
"I would note that Russia has said it expects the Assad regime to deliver a substantial portion of its chemical weapons stockpile in the relatively near future. And we obviously believe that's very important," Carney said.
Carney added that Russia "obviously has a great deal at stake" in the Syrian government fulfilling its responsibilities under the U.S.-Russian agreement.
Deputy Foreign Minister Faisal al-Meqdad said on Wednesday Syria was trying to meet its obligations.
"Syria is proceeding with all determination, strength and credibility to fully implement the agreements with the U.N.-OPCW," the Syrian national news agency SANA quoted him as saying.
In an apparent reference to clearing a road through disputed territory to the northern port of Latakia for shipment abroad, Meqdad said "there can be no leniency at all when it comes to transporting chemical weapons out of Syria."
British Prime Minister David Cameron said he was worried that the chemical weapons handover was behind schedule, and British diplomats said they planned to raise the matter at the United Nations Security Council on Thursday.
"Britain will continue to put pressure on all parties to make sure the chemical weapons are produced and destroyed," Cameron told parliament in London.
Syria had already missed a December 31 deadline to relinquish the most poisonous chemical agents, including mustard gas and sarin precursors.
So far, Syria has transported slightly more than 4 percent of the 1,300 metric tons it reported to the OPCW. The two small shipments of chemicals are being stored on a Danish vessel in the Mediterranean.
Under the U.S.-Russian agreement, prompted by a sarin gas attack near Damascus that killed hundreds of civilians, Syria has until June 30, or another five months, to completely eliminate its chemical weapons program.
Washington blames the poison attacks on the Assad government and threatened military retaliation.
Damascus has blamed the delay on security problems and the threat of attacks by rebels on road transports to the northern port of Latakia. It has requested additional armor and communications equipment.
But the United States and the United Nations, which is jointly overseeing the destruction program with the OPCW, said last week Syria has all the equipment it needs to carry out the operation and should proceed as quickly as possible.
The next major deadline is March 31, by when the most toxic substances are supposed to be destroyed outside Syria, on a special U.S. cargo vessel, the Cape Ray.
On Thursday, the head of the joint mission, Sigrid Kaag, will brief the United Nations about the operation in New York.
With the U.N. Security Council divided over imposing sanctions against Syria, some diplomats believe the threat of force may be the only way to get Assad to relinquish his weapons of mass destruction.
U.S. official said the use of force has never been taken off the table, but French Foreign Minister Laurent Fabius went a step further in recent comments.
He told Europe 1: "It's not on the agenda, but when you have a government ... when a government makes commitments before the international community, it must respect those commitments."
Asked if that was a warning, he replied: "Yes."
On the border with Turkey, Syrian families without passports were being turned away because a refugee influx caused by intensified "barrel bombing" in Aleppo filled up its camps, the Turkish Humanitarian Relief Foundation (IHH) said.
One of the Syrian opposition's most vocal allies, Turkey has taken in hundreds of thousands of Syrian refugees.
But resources have been stretched after Assad's forces intensified attacks on Aleppo, dropping barrel bombs and slowly winning ground against rebels weakened by weeks of infighting.
"Camps in Kilis are at full capacity unfortunately, but there are free spaces in our other camps," a press officer for Turkey's state AFAD disaster agency said.
Ankara is sticking to its "open border" policy and refugees will be accepted "following necessary security controls", the press officer said.
A camp inside Syria near the Syrian Bab al-Salam border crossing, 50 km (30 miles) north of Aleppo, is also full, IHH's Kilis media officer said, adding that numbers there had risen to 25,000 from 14,000 in the last week.
Turkish police at Oncupinar border post across from Bab al-Salam said restrictions applied to those without passports, but that the crossing was open, with no big crowd at the gate.
The use of barrel bombs - oil drums or cylinders packed with explosives and metal fragments dropped from helicopters - was condemned by Syria's opposition delegation and its Western backers at last month's peace talks in Switzerland.
Further east, the Syrian Observatory for Human Rights said that for the last 18 days Turkish authorities have prevented more than 2,000 refugees, including women and children, from crossing into Turkey after fleeing the city of Raqqa.