Turkey's prime minister vowed on Tuesday to retaliate against Syria for its 'heinous' downing of one of its warplanes, as the head of NATO said such an attack on one of its members was unacceptable.

Amid reports of clashes around elite Republican Guard posts in Damascus, Ankara raised the heat on the Assad regime by accusing it of shooting down the jet last Friday while it was in international airspace, and without warning.

The United States, meanwhile, denounced the United Nations' "colossal failure" to protect civilians inside Syria during a 15-month conflict which activists say has cost more than 15,000 lives.

Turkey's Prime Minister Recep Tayyip Erdogan, once a close ally of President Bashar al-Assad, has become one of the Syrian leader's biggest critics and his reaction to the downing of the jet represented his fiercest outburst to date.

"They acted without warning. This is a hostile act... a heinous attack," Erdogan told a parliamentary meeting of his ruling AKP party.

The premier admitted the Turkish plane had violated Syrian airspace but said it was only for a short time and "by mistake". He insisted it was not in Syrian airspace by the time it came under attack.

"Turkey will exercise its rights, born out of international law, with determination, and take the necessary steps by determining the time, place and method by itself," Erdogan told the lawmakers.

"The rules of engagement of the Turkish Armed Forces have changed given this new development," Erdogan added.

Any security or military risk posed by Syria on the Turkish border would be "considered a threat and treated as military target."

"This latest development shows that the Assad regime has become a clear and imminent threat to the security of Turkey, as well as for its own people," he added.

After a request from Turkey, NATO's secretary-general Anders Fogh Rasmussen hosted talks with ambassadors of the alliance's 28 members in Brussels where Turkey outlined the circumstances of the downing of its Phantom 4 fighter jet.

The two-man crew remain missing.

"Allies have expressed their strong support and solidarity with Turkey," Rasmussen told reporters after the meeting.

"We consider this act to be unacceptable and condemn it in the strongest terms.

"It is another example of the Syrian authorities' disregard for international norms, peace and security, and human life.

"We continue to follow the situation closely and with great concern," he added after talks lasting a little over an hour.

"Let me make this clear. The security of the alliance is indivisible. We stand together with Turkey in the spirit of strong solidarity," Rasmussen said.

Turkey requested consultations under Article 4 of NATO's founding treaty, enabling any of the allies to call for talks should they consider their territorial integrity, political independence or security under threat.

It was only the second time since the North Atlantic Treaty Organization was set up in 1949 that consultations have been requested under Article 4, the last time in 2003 also being at Turkey's request during the Iraqi war.

Syria has defended its downing of the jet, saying it was a response to "a gross violation" of its sovereignty.

"The Turkish warplane violated Syrian airspace, and in turn Syrian air defences fired back and the plane crashed inside Syrian territorial waters," foreign ministry spokesman Jihad al-Makdissi said on Monday.

NATO, which conducted a seven-month bombing campaign of Libya last year, has so far been notably reluctant to become sucked into the conflict in Syria.

And Iran, an ally of Turkey and Syria, called on Tuesday for regional countries rather than outside powers to help resolve the row.

Iran's foreign ministry spokesman Ramin Mehmanparast said that "key players in the region" would be able to prevent the incident escalating into a problem involving other countries.

"We will use our good relationship with the two countries to resolve the issue," he said.

-- Fierce clashes in Damascus suburbs --

In Syria itself, Syrian armed rebel forces and regime army units were locked in fierce clashes around elite Republican Guard posts in the suburbs of Damascus, according to pro-opposition activists.

"Violent clashes are taking place around positions of the Republican Guard in Qadsaya and Al-Hama," eight kilometres (five miles) from central Damascus, Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.

"This is the first time that the regime uses artillery in fighting so close to the capital," he said. "This development is important because it's the heaviest fighting in the area and close to the heart of the capital."

"These suburbs are home to barracks of troops which are very important for the regime like the Republican Guard. This is also where the families of (army) officers live," he said.

The sustained bloodshed in Syria saw Washington lashing out at the United Nations on Monday.

"The situation in Syria represents a colossal failure by the Security Council to protect civilians," Washington's UN ambassador Susan Rice said as the 15-nation body debated the crisis.

"It is a shame that this Council continues to stand by rather than to stand up," she said giving a withering assessment of the situation in Syria.

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