Russian President Vladimir Putin met Turkey’s prime minister for talks expected to cover their opposing views on the conflict in Syria, which Monday ruled out the use of chemical arms against its own people.
On the battlefield, artillery and aircraft battered rebel positions in and around Damascus on Monday in an operation to secure the capital, a monitoring group said.
Putin landed in Istanbul and went straight into talks with Turkish Prime Minister Recep Tayyip Erdogan, whose country is at loggerheads with Russia over how to tackle the crisis in Syria, despite growing trade and energy links.
Those tensions came to a head in October when Turkey intercepted a Syrian plane en route from Moscow to Damascus on suspicion it had military cargo, drawing an angry response from Russia.
Ankara said the cargo contained military equipment destined for the Syrian defence ministry. Moscow insisted it was dual-purpose radar equipment which was not banned by international conventions.
Turkey, once an ally of the Damascus regime, has become one of its fiercest critics. But Moscow remains one of President Bashar al-Assad’s few allies, routinely blocking resolutions against his regime in the UN Security Council.
Russia also objects to Turkey’s request to NATO for the deployment of Patriot missiles near its volatile border with Syria. It has warned such a move could spark a broader conflict that would draw in the western military alliance.
In Damascus, a foreign ministry spokesman said in a response to a warning from US Secretary of State Hillary Clinton that Syria will "never, under any circumstances" use chemical weapons against its own people.
"Syria confirms repeatedly it will never, under any circumstances, use chemical weapons against its own people, if such weapons exist," he said, quoted on state television.
After a New York Times report over detected movement of chemical weapons by the Syrian military, Clinton issued a warning to Damascus on Monday.
"This is a red line for the United States," the secretary of state said on the eve of a NATO meeting in Brussels. "Once again we issue a very strong warning to the Assad regime."
On Monday, an air strike killed least 12 people -- eight rebels and four civilians -- and wounded more than 30 in the northeastern town of Ras al-Ain, on the border with Turkey, the Syrian Observatory for Human Rights said.
The warplane hit the Mahata district "controlled by the (jihadist) Al-Nusra Front, Ghuraba al-Sham and other rebel battalions," the Observatory said.
The Syrian Observatory also reported that artillery gunners on Sunday night targeted the Damascus districts of Hajar al-Aswad and Tadamun as well as the Palestinian refugee camp of Yarmuk in the south of the capital.
The army bombarded Yabrud to the north, Yalda to the south and the Eastern Ghouta towns of Douma, Harasta, Irbin and Haran al-Hawamid, in the area of the road linking Damascus to its international airport, it said.
In the south, aircraft bombed Beit Sahem and its orchards as fierce clashes raged on the ground between troops and rebels, the Observatory said.
The pro-regime newspaper Al-Watan said: "To keep securing the road to Damascus international airport from the south, the army is continuing its drive in Al-Hujeira, Aqraba, Beit Sahem."
Forces loyal to Assad have been trying to establish a secure perimeter around Damascus at all costs, turning the province into one of the main battlegrounds in the country’s 20-month conflict.
Analysts say the objective is to put the regime in a position to negotiate a way out of the conflict that the Observatory says has cost more than 41,000 lives since March 2011.
In central Syria, the Britain-based Observatory also reported clashes with rebels since Sunday in the central city of Hama, prompting authorities to send in reinforcements.
The Observatory, which relies on a network of activists and medics in civilian and military hospitals, said a total of 134 people -- 58 civilians, 41 soldiers and 35 rebels -- were killed in countrywide violence on Sunday.