Moscow has accused Ankara of endangering Russian lives after Turkey forced a Syrian passenger plane to land and seized what it suspected was military equipment being ferried from Russia to Syrian President Bashar al-Assad.
Damascus said on Thursday the interception of the Syrian Air plane was an act of piracy, further heightening tensions between the neighbours after Turkey's chief of staff warned his troops would respond with greater force if shells from Syria continued to hit Turkish territory.
Military jets escorted the Damascus-bound Airbus A-320,which was carrying around 30 passengers from Moscow, into Ankara airport late on Wednesday after Turkey received intelligence that it was carrying "non-civilian cargo".
Russia, which has stood behind Assad's government during an 18-month-old uprising that has killed some 30,000 people, angrily demanded an explanation.
"The lives and safety of the passengers were placed under threat", the Russian Foreign Ministry said in a statement, adding that 17 of its nationals on board were refused access to Russian diplomatic staff.
Turkey said it had acted within international law.
"We are determined to control weapons transfers to a regime that carries out such brutal massacres against civilians. It is unacceptable that such a transfer is made using our airspace," Ahmet Davutoglu, Turkey's foreign minister, said.
"We received information this plane was carrying cargo of a nature that could not possibly be in compliance with the rules of civil aviation," he said in Athens during an official visit.
Meanwhile, Mahmoud Said, Syria's transport minister, accused Turkey of "air piracy", according to Lebanon's Al-Manar television station.
The country has also closed its airspace to Turkish planes, a development that followed Ankara's declaration that Syrian airspace was "unsafe" for Turkish aircraft.
Russia is one of the closest allies of Assad's government and has blocked several UN resolutions against Damascus.
"Once a week, a Syrian Airlines airplane flies from Moscow bound for Damascus," the Interfax news agency quoted Vnukovo Airport spokeswoman Yelena Krylova as saying. "The plane took off normally, there were no incidents."
An unidentified "Russian arms export source" told Interfax that there were no weapons or military equipment aboard.
Assad described Syria and Turkey as "brothers" in an interview published Thursday, saying Turkey "has no reason to go to war" over recent cross-border clashes.
"We should work on this issue together," he told left-leaning Turkish newspaper Aydinlik , after a week of shelling between the neighbours left several dead.
"In times like this, countries should correct their mistakes by talking to each other. [The] Turkish public is noble. We have no problems with the Turkish people and the Turkish soldiers. Syria is not an enemy to Turkey. We've always known Turkey as brothers."
But the interview, which appeared to be an effort to calm tensions, also carried a rebuke.
"We have problems with the Turkish government," said Assad. "We're having problems along the Turkish border because of the attitude of the Turkish government. [The] Turkish government's also responsible for the deaths. And the reason the relationship has come to this point is also a fault of the Turkish government. Not the Turkish people."
The comments were published a day after the grounding of the Syrian passenger jet intercepted en route from Moscow to Damascus
The airliner, carrying some 30 passengers, was allowed to leave Turkey and arrived at its destination.
On Wednesday, Turkey's military chief pledged to keep up the pressure on its southern neighbour - remarks that came a day after NATO said it stood ready to defend Turkey, amid fears of a regional escalation of the conflict.
"We responded and if [the shelling] continues, we will respond with more force,'' General Necdet Ozel told reporters after inspecting troops deployed to the 910km border.
Turkey has reinforced the border with artillery and also deployed more fighter jets to the area.
Meanwhile, battles between government forces and opposition fighters continue across Syria.
Ban Ki-moon, the UN secretary-general, urged the Assad administration to declare a unilateral ceasefire, calls which Syria rejected .
Instead, clashes intensified near the Syrian border town of Azmarin and heavy machinegun fire could be heard from Turkey.
Scores of civilians crossed a narrow river marking the border as they fled the fighting in Azmarin and surrounding villages.
Residents from the Turkish village of Hacipasa helped pull them across in small metal boats.
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"The firing started getting intense last night. Some people have been killed, some are lying wounded on the road," said Mune, a 55-year-old woman who fled Azmarin.
"People want to escape but they can't. Many have settled in a field outside the town and are trying to come."
The UNHCR says more than 90,000 Syrians are living in camps in southern Turkey.
Hillary Mann Leverett, senior foreign policy lecturer at the American University in Washington, DC, told Al Jazeera that the "increasingly militarised" tension between Turkey and Syria is "very, very destabilising", with potential for NATO to get involved.
"The precedent is certainly there. NATO got involved pretty quickly in Libya, and the plans are there to become involved in Syria," said Mann Leverett.
US Defence Secretary Leon Panetta said on Wednesday that a team of US military planners is in Jordan to help the Amman government grapple with Syrian refugees, bolster its military capabilities and prepare for any trouble with Syria's chemical weapons stockpiles.
Source: Al Jazeera and agencies