March will be a "critical" month for Syria if it is to maintain its timetable for dismantling its chemical weapons arsenal, according to the United Nations official entrusted with overseeing the mission.
The remarks by Sigrid Kaag, special coordinator for a joint mission by the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW), came as the US accused Syria of stonewalling OPCW members and refusing to seriously negotiate on the destruction of its facilities used to produce poison gas.
"The month of March, as I informed the Security Council, is the critical month to look at continued progress towards the overall deadline," Kaag said.
She made her remarks after briefing members of the UN Security Council early on Wednesday by Syria towards the goal of destroying or handing over its arsenal of banned weapons before a June 30 deadline.
In a separate interview to Al Jazeera's Diplomatic Editor James Bays, Kaag said all parties involved in Syria's disarmament were focused on the job.
After Syria missed several key dates, the Security Council last week demanded that it move faster.
The Syrian government blames the delays on insecurity in the country, where it is locked in a struggle with rebels seeking the overthrow of President Bashar al-Assad.
Prior to a shipment on Monday, the US estimated that Syria had shipped out just five percent of its stockpile.
Kaag said, however, that there was "an acceleration and an intensification" of effort by Syria, and that about 35 percent of weapons material has now been shipped.
"A number of shipments have taken place and will continue to take place," she said.
"About one-third of Syrian chemical weapons materials has been removed or destroyed."
Over the next few days, she said, "we expect to reach already 40 or 41 percent, and we look forward to see continued progress".
Kaag declined to comment on a UN human rights investigators' report that said chemical weapons used in two incidents in Syria last year appear to have come from the stockpiles of the Syrian military.
Last year Syria had asked the OPCW for permission to convert for peaceful use some of the facilities declared under its weapons programme, but diplomats said they were reluctant to accept such a plan as it could leave Syria with a residual chemical weapons capability.
In comments on her Twitter feed, Samantha Powers, US ambassador to the UN, said: "OPCW trying to reach agreement to destroy CW production facilities-#Syria refusing to seriously negotiate & is (about) to miss another deadline."
In another statement on Twitter, she said: "#Syria must accelerate process to comply with @OPCW deadlines-only 20% of priority 1 chemicals removed so far. Delays are dangerous."
Priority 1 chemicals are the deadliest precursors for poison gas.
Bashar Jaafari, Syria's UN envoy, did not respond to a request for comment.
The US State Department announced on Wednesday that Jaafari would from now on be confined to a 25-mile radius from central New York City.
Assad agreed to destroy his chemical weapons following global outrage over a sarin gas attack in August that killed more than 1,000 people, many of them children.
The world's deadliest chemical attack in 25 years, it drew a US threat of military strikes that was averted after Assad pledged to give up his chemical arms.
Forces closing in
In Syria, meanwhile, government forces are closing in on one of the last rebel strongholds near the border with Lebanon. Yabroud, which lies north of Damascus, is a strategically important town near the border with Lebanon.
Videos posted by opposition activists show a helicopter dropping bombs on the city, as well as the neighbouring village of Sahel.
Earlier this week, forces loyal to Assad, assisted by fighters from Lebanon's Hezbollah, captured Sahel. It is one of a number of towns Hezbollah seized in the region since launching an offensive there in November.
Meanwhile, in Syria's largest city, Aleppo, the army is continuing to strafe rebel-held areas.
More than 130,000 people have been killed in Syria's three-year-old civil war.