As fighting raged on the ground, a war of words between Syria and Turkey grew angrier after Ankara said military supplies were on an airliner it intercepted en route from Moscow to Damascus.
And 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 AFP.
The Syrian foreign ministry accused Turkish Prime Minister Recep Tayyip Erdogan of lying when he said the jet intercepted on Wednesday was carrying "equipment and ammunition shipped to the Syrian defence ministry" from Russia.
Sergei Lavrov, Moscow's foreign minister, said the cargo was legal, in Russia's first remarks about the incident.
"This cargo is electrical technical equipment for radar stations, this is dual-purpose equipment but is not forbidden by any international conventions," Lavrov said.
"There were no weapons on board this plane, nor could there have been," he said in remarks posted on the Kremlin website.
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.
-- One of the deadliest days --
UN-Arab League peace envoy Lakhdar Brahimi is also due in Istanbul on Saturday after talks with Saudi Arabia's King Abdullah on Friday.
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 more than 348,000 refugees in neighbouring countries.
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, regime 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 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.
Rebels told another AFP correspondent in Aleppo that their forces captured a regime military site near the city early on Friday. Massive clouds of grey smoke could be seen rising from the site in Al-Taana.
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 insurgents 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.
Despite the ongoing violence, anti-regime demonstrations were held across Syria after the weekly Muslim prayers.
In Aleppo, regime 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.