At least 18 Syrian soldiers were killed Wednesday in a car bombing near the Turkey border, the opposition said, a day before international mediator Lakhdar Brahimi was to visit Damascus.
Russia, meanwhile, accused Western members of the UN Security Council of supporting terrorism by failing to condemn attacks on Syrian government forces.
The opposition Syrian Observatory for Human Rights said the bombing took place near a military outpost in the northern area of Saraqab, close to the border with Turkey.
Russian Foreign Minister Sergei Lavrov told reporters in Kazakhstan that "an act of terror is an act of terror. If it looks, is planned and carried out like an act of terror, it is an act of terror."
Russia, a staunch ally of President Bashar Assad, has vetoed with China three UN resolutions condemning the regime's crackdown on protesters.
The Syrian Foreign Ministry said that Brahimi, a veteran Algerian diplomat, would meet Thursday with Syrian Foreign Minister Walid al-Moallem in Damascus. There was no word yet if he would also meet Assad.
Opposition activists said at least 76 people were killed Wednesday across Syria.
Clashes between Assad's soldiers and rebel forces erupted near an airport in the northern city of Aleppo. Activists said a convoy carrying Syrian Christian Armenians from Aleppo came under attack, leaving four dead and 14 wounded.
Aleppo, the country's commercial hub, has a large Syrian Christian Armenian community. The battle for Aleppo started in mid-July.
With the civil war in its 18th month and with little movement on the diplomatic front, Brahimi has attended a series of meetings aimed at finding a solution to the conflict.
Brahimi, who replaced former UN chief Kofi Annan, on Wednesday met Arab League chief Nabil al-Arabi and Qatari Prime Minister Hamad bin Jassem, who leads an Arab ministerial committee on Syria.
A spokesman for the main opposition Syrian National Council (SNC), George Sabra, told dpa that earlier this month Brahimi met SNC officials in France who had briefed him on the situation in Syria.
"We told him that the regime has to leave before any dialogue or talks on a transitional period in Syria," Sabra said.
The United Nations says an estimated 1.2 million Syrians, more than half of them children, are internally displaced, while about 250,000 refugees have sought shelter in neighboring Jordan, Lebanon, Turkey and Iraq.
Opposition activists say more than 27,000 people have died since the unrest began in March 2011.
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