NATO's governing body will meet Tuesday to discuss Syria's downing of a Turkish jet, in what Turkish leaders insist was international airspace, a spokeswoman for the alliance has confirmed.

Turkey has been calling on NATO to discuss a response to last Friday's incident, which has deepened the regional turmoil caused by the conflict in Syria, where reports Sunday said more than a dozen people had died in the latest clashes between rebels and government troops.

The plane wreckage was found Sunday in the Mediterranean at a depth of 1,000 metres, Turkish state-run TRT television said. The pilots are unaccounted for.

Turkish Foreign Minister Ahmet Davutoglu said the jet was on a training flight to test Turkey's radar capabilities, not spying on Syria. He said the plane mistakenly strayed into Syrian airspace Friday, but was quickly warned to leave by Turkish authorities and was a mile inside international airspace when it was shot down off the coast of Latakia.

Syria insisted Saturday that the shooting was "not an attack" and that the plane had violated its airspace.

'The Assad regime should not make the mistake of believing that it can act with impunity. It will be held to account for its behaviour.'—British Foreign Secretary William Hague

TRT reported Davutoglu had called for a special meeting of NATO on Tuesday to discuss Article 4 of the NATO charter in relation to Friday's incident. The article says member countries "will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the parties is threatened."

The plane's downing drew international concern, including from countries pressuring Syrian President Bashar al-Assad to step aside due to the bloodshed in his country over the past 15 months. Opposition activists say Assad's crackdown on an increasingly armed popular uprising has killed 14,000 people, most of them civilians.

British Foreign Secretary William Hague said Sunday he was "gravely concerned by the Syrian regime's action in shooting down" the plane and that Davutoglu had told him no warning was given.

"This outrageous act underlines how far beyond accepted behaviour the Syrian regime has put itself, and I condemn it wholeheartedly," Hague said in a statement. "The Assad regime should not make the mistake of believing that it can act with impunity. It will be held to account for its behaviour."

Talk of international summit

Hague met last week with United Nations and Arab League special envoy Kofi Annan for talks on plans for an international summit, while British officials discussed the issue in Geneva on Saturday with members of Annan's team. Hague noted Sunday that "The UK stands ready to pursue robust action at the United Nations Security Council."

In Syria, state-run SANA news agency said gunmen infiltrating from Turkey clashed with Syrian border guards in Rabiah, a region in the coastal Latakia province. SANA said several infiltrators died in the late Saturday clash, while others reportedly returned to Turkey. It said several Syrian border guards were wounded, but didn't specify how many.

Turkey denies sheltering armed Syrian rebels, although many Syrian refugees have fled to camps on the Turkish side of the border.

Syrian activists, meanwhile, said rebels captured a military base in the northern Syrian province of Aleppo, confiscating large amounts of ammunition. The Britain-based Syrian Observatory for Human Rights said 16 government troops died in the attacks on the base near the rebel-held town of Daret Azzeh and nearby checkpoints early Sunday.

Area activist Mohammed Saeed said the rebels had removed hundreds of artillery shells from the base. Saeed added via Skype that troops retaliated with intense shelling on the area using helicopter gunships.

On Friday, state media said 25 people were kidnapped by "terrorists" and killed in Daret Azzeh. Activists said the 25 killed were pro-regime gunmen known as shabiha.

Also on Sunday, Syrian opposition groups met in Brussels to hash out differences and plan for a democratic transition. The disparate groups are divided over whether outside military intervention would help or hurt and whether to engage in dialogue with Assad's regime. The conference, attended by some 50 people, will continue Monday.

Jordanian Information Minister Sameeh Maaytah, meanwhile, said Sunday that three other Syrian pilots had defected last week, even before a pilot flew his warplane into neighbouring Jordan. He said the other three crossed overland into Jordan. He was unsure if the four pilots knew each other or had co-ordinated their escape from Syria.