Syrian rebels killed 21 members of the elite Republican Guard in Damascus province on Thursday, as the army subjected several districts of second city Aleppo to fierce shelling, a monitoring group said.
The shelling in Aleppo took place a day after a string of explosions in the heart of the city left 48 people dead, in an attack claimed by the shadowy Islamist group Al-Nusra Front.
The foreign ministry wrote to the UN Security Council and to Secretary General Ban Ki-moon urging condemnation of the attacks, state television said.
"We look forward to the UN Security Council's sincere condemnation of the terrorist attacks that struck Aleppo, and of those who stand behind them," said the letter, which blamed foreign-backed "terrorists" for the attacks.
"We consider this a test of the international community's credibility, and of its intentions to fight terrorism."
The Syrian Observatory for Human Rights said the toll of Republican Guards killed in the town of Qudsaya, west of Damascus, may rise beyond 21, given the ferocity of the violence.
"The rebels likely used a small explosive device, but it was placed near the Republican Guards' living quarters in Qudsaya," said Observatory director Rami Abdel Rahman, who added that clashes were continuing after the blast.
The attack occurred a day after the army launched a major offensive on Qudsaya and a neighbouring locality, the Observatory and AFP journalists said.
Also in Zabadani, northwest of Damascus, the army and rebels clashed, said the Observatory.
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"The quality of operations staged by the rebels is improving," said Abdel Rahman. "This is especially noticeable in Damascus province, which is strategic for both the rebels and the regime because of its proximity to the capital."
Amid the violence, regime forces carried out a large number of arrests in Qudsaya, the Observatory said.
In Aleppo, the army shelled the Nayrab, Salaheddin, Mashhad, Bab al-Nasr and Sakhur districts, said the Observatory. The districts are spread across the city, with Salaheddin in the southwest and Sakhur in the northeast.
At least 28 people were killed across Syria on Thursday, according to a preliminary Observatory toll, a day after 236 people died in violence.
More than 31,000 people have been killed since the outbreak in March last year of an anti-regime revolt, which began as peaceful protests for reform but morphed into an armed insurgency when demonstrations were brutally crushed.
Most rebels, like the population, are Sunni in a country dominated by a minority Alawite regime.
Meanwhile, Amnesty International called on the Syrian authorities to either charge or release two human rights activists detained in Damascus earlier in the week.
Lawyer Khalil al-Maatouq and his colleague Mohammed Thatha were arrested while on their way to work on Tuesday, said their employer, the Syrian Centre for Legal Studies and Research.
"The families of these men have a right to know what has happened to them," said Ann Harrison, deputy director of Amnesty's Middle East and North Africa Programme.