Syria banned Turkish passenger flights from its airspace Sunday in a retaliatory move, as its troops launched a counter-attack on rebels in the country's north in a bid to regain lost ground.
The reprisal over Turkey's confiscation of what Russia said was radar equipment being flown from Moscow to Damascus came despite a flurry of diplomacy intended to calm soaring tensions between the neighbours.
Syria accuses Turkey of channelling arms from Gulf Arab states to rebels fighting its troops, who have been under mounting pressure across large swathes of the north, including Aleppo.
The flight ban went into force from midnight (2100 GMT Saturday) "in accordance with the principle of reciprocity", SANA state news agency said.
Meanwhile, Turkey banned all Syrian aircraft from its air space on Sunday.
Asked if Syrian aircraft were now banned from Turkish air space, a Foreign Ministry official said: "Yes, civilian aircraft. Military aircraft were de facto banned way before."
Since last Wednesday, Turkey had warned its airlines to avoid Syrian airspace for fear of retaliation for that day's interception of the Syrian Air passenger plane by Turkish jets on the allegation it was carrying military equipment.
Ankara has taken an increasingly strident line towards its southern neighbour since a shell fired from the Syrian side of the border killed five Turks on October 3.
It has since repeatedly retaliated for cross-border fire, prompting growing UN concern and a flurry of diplomatic contacts.
After talks with his German counterpart Guido Westerwelle on Saturday, Turkish Foreign Minister Ahmet Davutoglu reiterated that Ankara would not tolerate any further border incidents.
"We will hit back without hesitation if we believe Turkey's national security is in danger," he said.
With the violence raging, UN and Arab League peace envoy Lakhdar Brahimi on Sunday headed to Iran, the Syria government's closest ally, after talks in Saudi Arabia and Turkey, the leading backers of the rebels.
Mr. Brahimi is on his second tour of the region after taking up his post at the start of September, replacing former UN chief Kofi Annan who quit complaining that he had not received sufficient support from the major powers.
On the battlefield, the military used fighter jets to bombard Maaret al-Numan, captured by the rebel Free Syrian Army earlier in the week, said the Syrian Observatory for Human Rights.
East of Maaret al-Numan, troops tried to block a new rebel assault on Wadi Deif army base – the largest in Idlib province of northwest Syria, much of which is in rebel hands – where large quantities of fuel and tanks are stored.
Battles broke out in Maarshurin and Hish, near the base, while warplanes bombarded the area, the Britain-based Observatory said.
The counter-strike came as rebels captured three army officers in Idlib, said the Observatory which relies on a network of activists, lawyers and medics on the ground inside Syria for its information.
SANA, meanwhile, reported that troops killed and captured "dozens of terrorists and destroyed anti-tank rockets" in the northern city of Aleppo, the country's commercial capital.
Elsewhere, two explosions hit the upscale district of Mazzeh in Damascus early on Sunday, the Observatory said, adding that one of them targeted the car of a pro-regime lawyer who was critically wounded.
SANA said the other blast was a "suicide attack" that caused no casualties.
In Damascus province, regime forces took back control of a military base in Atibah, a day after rebels had seized it.
At least 18 people were killed nationwide on Sunday, the Observatory said, in addition to more than 33,000 who have died since the revolt against the regime erupted in March last year.
Human Rights Watch, meanwhile, urged the Syrian military to stop using cluster bombs, weapons that can contain up to 650 submunitions which are sprayed over a large area before exploding.
"Syria's disregard for its civilian population is all too evident in its air campaign, which now apparently includes dropping these deadly cluster bombs into populated areas," said Steve Goose, arms director at the New York-based group.
With a report from Reuters