Syria on Saturday warned the United Nations that rebels may use chemical weapons after they gained control of a factory producing toxic chlorine, dpa reported.
The statement came after the United States and Britain expressed fears that the Syrian regime might use chemical weapons as rebels pushed their fight towards the capital Damascus.
"Terrorist groups may resort to using chemical weapons against the Syrian people ... after having gained control of a toxic chlorine factory," the Syrian Foreign Ministry said.
Activists based in Aleppo confirmed to dpa that rebels linked with the al-Nousra Front had taken control of the Syrian-Saudi Chemicals Company (SYSACCO) factory near the town of Safira, east of the northern city of Aleppo.
The Foreign Ministry statement reiterated that Syria would "not use chemical weapons under any circumstance, if they exist."
Syria "is defending its people against terrorism, which is supported by known countries, with the United States at the forefront," it added.
British Foreign Secretary William Hague said there was evidence that the Syrian government could use its chemical weapons stocks in quelling the revolt in the country.
"We are extremely concerned about the stockpiles of chemical and biological weapons, and we are also concerned about evidence during the last couple of weeks that the regime could use them," Hague told reporters while attending a security conference in Bahrain Saturday.
He added that the option of military intervention in Syria had not been "ruled out", but Britain continued to seek a peaceful transition in the war-torn country.
On the ground, troops sealed off the entrances of Damascus after rebels attacked checkpoints in various parts of the city, activists said.
"Around 13 army checkpoints were attacked at the entrances of Damascus. This prompted troops to close areas, especially those leading to the eastern, western and southern suburbs," Haytham al-Abdullah, an activist based in Damascus, told dpa.
Rebels are attempting to seize Damascus airport and a main road leading to it in order to cut supply lines to government troops. The two sides have been fighting near the facility for more than a week.
Activists reported that dozens of rebel fighters were killed in the nearby area of Daraya.
"There was renewed heavy shelling on several areas of Daraya, which is experiencing a major military operation by troops trying to seize control of the town and its surroundings," the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman said.
State television reported that troops killed hundreds of rebels in Daraya, without giving details.
A source close to the regime said most of those killed were Arab insurgents linked to al-Qaeda and had come from neighbouring countries like Turkey and Lebanon.
News from Syria is hard to verify independently as authorities have barred most foreign media from restive areas since the uprising against the rule of President Bashar al-Assad started in March last year.
Military commanders of several brigades from the rebel Free Syrian Army have agreed to unify their command.
They selected a 30-member supreme military council, led by General Salim Idriss, in preparation for a decisive battle in Damascus, said activists.
The head of the National Coalition of Syrian Revolutionary and Opposition Forces, Mouaz al-Khateeb, is to join a meeting of European Union foreign ministers in Brussels on Monday, a move aimed at promoting the status of the opposition grouping.
Al Khateeb, in a statement aired on the Dubai-based Al Arabiya broadcaster, rejected any plans to divide Syria and called for unity among the various sects in order to topple the regime "very soon."
"Negotiations are underway between opposition figures to form a transitional Syrian government," he said, following talks held in Cairo earlier Saturday.
The US and other Western powers have been pressuring the coalition, launched in Qatar last month, to show unity and coordinate with forces fighting to oust al-Assad's regime.
Meanwhile, the United Nations High Commissioner for Refugees on Saturday said that more than 150,000 Syrian refugees have fled the conflict in their home country for Lebanon.
"There are around 109,000 refugees formally registered within the agency and 41,000 others currently contacting it to be registered," a statement released by the UN said.