Turkish Prime Minister Tayyip Erdogan says a Syrian passenger plane forced to land in Ankara was carrying Russian-made munitions destined for Syria's armed forces, ratcheting up tension with his country's war-torn neighbour.
Damascus said the plane had been carrying legitimate cargo and described Turkey's actions as an act of "air piracy", while Moscow accused Ankara of endangering the lives of Russian passengers when it intercepted the jet late on Wednesday (local time).
The grounding of the plane was another sign of Ankara's growing assertiveness towards the crisis in Syria. Turkey's chief of staff warned on Wednesday the military would use greater force if Syrian shells continued to land in Turkey.
"This was munitions from the Russian equivalent of our Mechanical and Chemical Industry Corporation being sent to the Syrian Defence Ministry," Erdogan told a news conference.
A spokeswoman for Moscow's Vnukovo airport told state news agency Itar-Tass everything put on the plane had cleared customs and security checks and no prohibited items were on board.
Asked about Erdogan's statement, the Russian Foreign Ministry referred to her remarks and declined further comment.
Russia's arms export agency said it had no cargo on the flight, and the Interfax news agency quoted a Russian diplomat as saying the cargo seized by Turkey was not of Russian origin.
Syrian Arab Airlines chief Ghaida Abdulatif said in Damascus the plane had been carrying civilian electrical equipment.
Turkey has become one of Syrian President Bashar al-Assad's harshest critics during a 19-month-old uprising that has killed some 30,000 people, providing sanctuary for rebel officers and pushing for a foreign-protected safe zone inside Syria.
Russia has stood behind Assad and an arms industry source said Moscow had not stopped its weapons exports to Damascus.
Military jets escorted the Airbus A-320, carrying around 30 passengers, into Ankara airport after Turkey received an intelligence tip-off. The Turkish foreign ministry said the plane had been given a chance to turn back towards Russia while still over the Black Sea, but the pilot chose not to do so.
"This hostile and deplorable Turkish act is an additional indication of the hostile policy of Erdogan's government," Syria's foreign ministry said in a statement, accusing Ankara of "harbouring terrorists" and allowing them to infiltrate Syria.
The Syrian conflict threatens to suck in neighbouring states and exposes the deep Sunni-Shi'ite rift in the Middle East.
Two Sunni Islamist rebel groups said late on Thursday they had detonated bombs in a state security compound in central Damascus. Lebanon's Shi'ite Hezbollah group - which, like Syria's rulers, is allied with Shi'ite Iran - meanwhile denied sending fighters to aid Assad.
Russian President Vladimir Putin had been expected to visit Turkey at the start of next week but Turkish officials said hours before the plane was grounded that Russia had requested the visit be postponed, citing his heavy work schedule.
"BRUTAL MASSACRES"
Turkey said it would stop more Syrian civilian aircraft using its airspace if necessary and instructed Turkish passenger planes to avoid Syrian airspace, saying it was no longer safe.
"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," Foreign Minister Ahmet Davutoglu said.
Turkey has boosted its troop presence along the 900-km (560-mile) border and returned fire in response to shelling from northern Syria, where Assad's forces have been battling rebels.
Chief of Staff General Necdet Ozel said on Wednesday his troops would respond "with greater force" if Syrian shelling continued and parliament last week authorised the deployment of troops outside Turkish territory.
Such approval has in the past been used for strikes against Kurdish militant bases in northern Iraq. In 2008 Turkey sent 10,000 troops backed by air power over the border.
Some 25 fighter planes were sent to a military base in the southern city of Diyarbakir, around 100 km from the Syrian border, on Monday, the Dogan news agency said.
Syrian refugees fleeing across a river into Turkey spoke of chaos as Syrian government forces battled rebels for control of the area around their home town of Azmarin on Thursday.
Loudspeakers in Azmarin, audible from Hacipasa on the Turkish side, called on rebel fighters to give up.
"Give up your weapons. Come and surrender. We are coming with tanks and planes," they said between bursts of mortar fire.
RISKS OF DEEPER INVOLVEMENT
Turkey has made clear that beyond like-for-like retaliation it has no appetite for unilateral intervention in Syria. Such a move would be fraught with risks.
Turkey relies on Russia, which has blocked tougher U.N. resolutions against Damascus, both for energy needs and to help realise its ambitions to be a hub for energy supplies to Europe.
Many Turks see Russia as harbouring sympathy towards the militant Kurdish Workers Party (PKK), which has stepped up violence in southeast Turkey in recent months. Turkish officials believe Syria and Iran have also been backing the group.
"We get 80 percent of our natural gas from Iran and Russia. Already the PKK card is being used by Iran against Turkey ... so the risks for Turkey of being involved in even a limited operation are huge," Ulgen said.
The establishment of foreign-protected safe zones in Syria would be hazardous, with the exit strategy for foreign forces dependent on the Syrian opposition's ability to topple Assad.
The opposition is deeply divided. Organisers of a Qatar conference aimed at uniting it said on Thursday it had been postponed until they can agree on fair representation for disparate groups.
The Syrian rebels are outgunned by the government but can still strike at will, while Assad has assumed personal command of his forces, convinced he can prevail militarily.
"The earlier Bashar goes, the easier the transition in Syria will be," French President Francois Hollande said on Thursday.
"The longer it lasts, the greater the risk of civil war, chaos and partition. I refuse to accept that."
Rebels attacked a Syrian army base near the main northern highway on Thursday to try to consolidate their control over the supply line to Aleppo, days after capturing a strategic town in the area, opposition activists said.
They used at least one tank seized from the army, as well as rocket-propelled grenades and mortar bombs, to hit the Wadi al-Deif base, three km east of the town of Maarat al-Nuaman, which they captured this week, they said.
