Turkey on Sunday accused Syria of shooting down one of its warplanes in international airspace, as a watchdog said the Damascus regime suffered new setbacks and violence reached new heights.
"According to our conclusions, our plane was shot down in international airspace, 13 nautical miles from Syria," Turkish Foreign Minister Ahmet Davutoglu told TRT television.
Syria has said it downed the F-4 phantom jet on Friday after it violated its airspace, and on Saturday Turkey acknowledged the plane may have done so in comments seen as a bid to cool tensions between the former allies.
Meanwhile, at least 20 people were killed, including 16 troops, in clashes with rebel fighters in Syria on Sunday, the Syrian Observatory for Human Rights reported.
"The clashes happened almost simultaneously at dawn," in the northern province of Aleppo bordering Turkey, the Observatory's Rami Abdel Rahman of told AFP.
The fighting took place in the town of Dara Aza, and at military checkpoints near the town of Al-Atarib and the village of Kafr Halab, the Britain-based watchdog said.
It reported that following an attack on an artillery battalion, a number of soldiers defected, taking with them a large quantity of weapons.
And, in another setback for the embattled regime of President Bashar Al-Assad, rebels captured 11 government soldiers in the central province of Damascus, it added.
"This is one of the bloodiest weeks in the conflict," Abdel Rahman said.
According to the Observatory's figures, 94 people were killed in Syria on Monday, 62 on Tuesday, 88 on Wednesday, 168 on Thursday, 116 on Friday and 116 on Saturday.
"It's like we are in a war," Abdel Rahman said. "Sometimes when two countries are at war, not even 20 people are killed a day. But now in Syria it has become normal to have 100 killed each day."
The mounting death toll was a result of the international community's inability to agree on a way to resolve the crisis, he said.
"The UN observers have suspended their mission and this is a very bad decision. They are just staying in Syria and not going out to observe," said Abdel Rahman.
"Either go out and observe properly or leave," he said, his frustration palpable.
Sunday's developments come after a bad week for the Assad regime.
On Thursday, a Syrian pilot was granted asylum in neighbouring Jordan after flying his MiG-25 fighter across the border.
Meanwhile, Iran, Assad's staunchest ally, called on Damascus and Ankara to show restraint after the shooting down of the Turkish warplane by Syria.
Tehran "asks both sides to show calm and restraint, and hopes that with tact and tolerance and dialogue this issue will be evaluated and through a peaceful resolution, tranquillity and stability will be preserved in the region," Foreign Minister Ali Akbar Salehi was quoted as saying.
He made the appeal in a telephone conversation late Saturday with Davutoglu.
Anti-aircraft batteries hit the jet as it flew low and fast about a kilometre (less than a mile) from the coast over Syrian territorial waters, a Syrian military spokesman said, adding it was unidentified at the time.
UN Secretary General Ban Ki-moon had expressed his "deep concern" over the incident in a telephone conversation with Davutoglu, a spokesman said.
NATO member Turkey acknowledged one of its warplanes may have violated Syrian airspace, but its muted response has been seen as a bid to take the tension out of the latest rift between the former allies.
Turkey-Syria relations have already been strained by Prime Minister Recep Tayyip Erdogan's outspoken condemnation of the Assad's regime's bloody crackdown, which rights activists say has killed more than 15,000 people since March 2011.
Meanwhile, a Russian ship that tried to deliver attack helicopters to Syria entered the northern port of Murmansk on Sunday after being forced to turn back when news of its mission was leaked.
An unnamed Russian diplomatic source said the ship would soon try again to make the highly controversial delivery under the Russian flag.
The switch appears to be an attempt to avoid security inspections that come when sailing under the flag of a third country.
Russia says the Soviet-era Mi-25 helicopters are being returned to Syria after undergoing repairs under a contract that could not be breached.
The Alaed was forced to turn back after its mission was initially mentioned by the US State Department and then reported in the British press.
Those reports prompted the ship's British insurer to withdraw coverage, effectively barring it from entering any other ports on its journey.