Syrian rebels captured a military airbase in the north on Friday and geared up for a major battle against loyalists as the opposition said it refuses to accept President Bashar al-Assad in talks on the 23-month conflict.
The rebels, from the Islamist Al-Nusra Front and the Muhajireen battalion, overran the base in Sfeira, east of Aleppo international airport, and captured a large stockpile of ammunition, the Syrian Observatory for Human Rights said.
The Britain-based watchdog also reported intermittent clashes around the Aleppo airport itself as well as around Nayrab airbase and another military complex, as the two sides squared up for a major fight.
"The army shelled the area around Aleppo international airport and Nayrab air base on Friday morning, while rebels used home-made rockets to shell Nayrab," Observatory director Rami Abdel Rahman said.
"The army is preparing a large-scale operation to take back control of Base 80," he added, referring to a military complex tasked with the security of both Nayrab and Aleppo airports.
Rebels seized the base on Wednesday after a battle that left at least 150 dead from both sides, among them senior army officers, according to the Observatory.
Regime tanks, meanwhile, shelled the town of Khan Sheikhun in the province of Idlib, killing at least 11 civilians, the Observatory said, adding that at least 107 people were killed in violence nationwide on Friday.
In Damascus, the army shelled the eastern district of Jobar where rebels have set up enclaves, the Britain-based group said.
It also said more than 100 civilians have been abducted in Idlib province in separate incidents, expressing alarm at what it called "sectarian kidnappings."
As they have done every Friday, Syrians protested across the country after weekly prayers to denounce what they called the "inaction" of the international community over the Syria crisis.
"In spite of you, O Bashar, we have our freedom!" chanted protesters in Sukkari, a rebel-held district of Aleppo.
State television broadcast footage of what it said was a pro-regime demonstration in Aleppo, and said residents had called for "armed men to leave their city."
On the political front, the opposition National Coalition said it refuses to accept Assad in any talks, as part of an eight-point "framework" it has drawn up for solutions to the conflict.
The Coalition issued the framework after a meeting in Cairo to discuss a proposal by its chief, Ahmed Moaz al-Khatib, for peace talks with regime representatives, a move that ruffled feathers in the umbrella opposition group.
"Bashar Assad and security leadership who are responsible for the current destruction of the country are outside the political process and must be held accountable for their crimes," it said in a statement issued in English.
Meanwhile, Syria has written to the United Nations blasting Turkey's "destructive" role in the conflict, state media reported.
"Turkey supports and publicly justifies terrorist, destructive acts" against Syria, the foreign ministry wrote in letters addressed to both the UN Security Council and Secretary General Ban Ki-moon.
"Turkey has turned its territory into camps used to house, train, finance and infiltrate armed terrorist groups, chief among them the Al-Qaeda network and the Al-Nusra Front."
The International Committee of the Red Cross in Geneva said the plight of civilians in Syria has reached catastrophic levels.
"After two years with no end to the military conflict, the situation of the civilian population has reached nothing short of catastrophic" proportions, ICRC Director of Operations Pierre Kraehenbuehl told reporters.
"There is no respite."
The ICRC has been aiding some 1.5 million Syrians, via the Syrian Arab Crescent, in a conflict that the UN says has left some 70,000 people dead.
The United Nations has also said the number of Syrians who have fled the country could hit 1.1 million by June.
Customs officers in Finland, meanwhile, said they had seized spare parts for tanks in a container en route from Russia to Syria on board a Finnish ship docked at Helsinki's Vuosaari port in January.
The European Union has banned all sales, delivery, transfers and exports of weapons to Syria.