The UN's envoy on Syria is to meet with Turkish Foreign Finister Ahmet Davutoglu in Istanbul to discuss the 19-month civil war in Syria.
Lakhdar Brahimi arrived from Saudi Arabia for the talks on Saturday amid growing tensions between Syria and its northern neighbor Turkey.
On Wednesday, Turkey intercepted a Syrian passenger plane en route from Moscow to Damascus and seized what it said was military equipment on board.
Syria denounced the move as air piracy and Russia has said the cargo was radar parts that complied with international law.
Davutoglu will also meet separately with Arab leaders and German Foreign Minister Guido Westerwelle on Saturday for talks on the Syria crisis.
Brahimi's visit comes after United States on Friday denounced Russia's policy of aiding the Syrian government as "morally bankrupt", pointing to the air-cargo incident.
Victoria Nuland, spokesperson for the US Department of State, refused to disclose what exactly had been in the cargo seized in Turkey on Wednesday, but said "we have no doubt that this was serious military equipment", aimed at bolstering the government of Bashar al-Assad, the Syrian president.
Sergei Lavrov, Russian foreign minister, however, said the Syrian Air plane intercepted by Turkey on a flight from Moscow to Damascus was carrying a cargo of dual-purpose radar equipment, and insisted Russia did not violate any laws.
"This cargo is electrical technical equipment for radar stations, this is dual-purpose equipment, but is not forbidden by any international conventions," Lavrov said.
Nuland acknowledged that Russia had not violated any embargo on the Damascus government, but said "the policy is still morally bankrupt".
"Everybody else on the Security Council is doing what it can unilaterally to ensure that the Assad regime is not getting support from the outside," she said in Washington.
"We have been saying for almost a year now, that no responsible country ought to be aiding and abetting the war machine of the Assad regime."
Turkey scrambled a fighter jet on Friday after a Syrian helicopter attacked the rebel-held town of Azmarin near the border, an official in Ankara told the AFP news agency.
The Syrian foreign ministry accused Recep Tayyip Erdogan, Turkish prime minister, of lying when he said the jet intercepted on Wednesday was carrying Russian military equipment.
Turkey's allies have warned of the risks embedded in the Syria conflict between the neighbours, which have exchanged cross-border fire amid fears the civil war could spark a regional conflagration.
Amid the growing alarm, German Foreign Minister Guido Westerwelle was due in NATO partner Turkey on Saturday for talks with his counterpart Ahmet Davutoglu.
"It is important that no one pours oil on the fire. We are counting on moderation and de-escalation," said Westerwelle.
Ahmad Fawzi, Brahimi's spokesman, said Brahimi and the king agreed "on the dire need to stop the bloodshed and provide humanitarian aid to the more than 2.5 million Syrians" affected by the fighting, and 348,000 plus refugees in neighbouring countries.
In-depth coverage of escalating violence across Syria
The Observatory said Thursday was one of the deadliest days since the anti-regime revolt erupted in March last year, with at least 240 people killed nationwide.
On Friday, government warplanes attacked two buildings in the Idlib town of Maaret al-Numan, where intense fighting has raged since rebels overran it on Tuesday after a fierce 48-hour battle, said the Britain-based Syrian watchdog.
Resupplying the army is "a fundamental aspect of the performance of the military," said Emile Hokayem, Middle East expert at the International Institute for Strategic Studies in London.
But he added that regime air supremacy was no longer decisive because the troops have "lost morale".
According to the Observatory, the rebels took 256 soldiers prisoner in capturing the town of Khirbat al-Joz and nearby areas in Idlib province along the border with Turkey since last week.
And an AFP reporter said the rebels, by seizing a stretch of highway near Maaret al-Numan, were able to cut the route linking Damascus to embattled commercial hub Aleppo on Thursday, choking the flow of troops to the north.
'Army base caputred'
Rebels told another AFP correspondent in Aleppo that their forces captured a government military site near the city early on Friday. Massive clouds of grey smoke could be seen rising from the site in Al-Tana.
In Aleppo itself, the head of the university hospital, who was accused by the opposition of backing the regime and kidnapped in July, was murdered and his body found on Friday, a friend of the doctor told AFP.
The opposition fighters also attacked a large air force post on the highway connecting Aleppo to Raqa province, further to the east, near Kweris military airport, the Observatory said.
Anti-Assad demonstrations were held across Syria after the weekly Muslim prayers.
In Aleppo, government forces fired on protesters in the Halab al-Jadida district, wounding a number of demonstrators, the Observatory said.
According to Observatory figures, more than 32,000 people have died in Syria since the revolt erupted on March 15, 2012.