Turkey has told the UN Security Council that Syria's downing of one of its fighter jets poses a "serious threat to peace and security," as Damascus insisted the plane had violated its airspace.
The United States meanwhile denounced the UN's "colossal failure" to protect civilians inside Syria as the death toll there continued to mount.
And a Syrian general was among a new group of officers and soldiers to defect and join the growing rebel ranks in Turkey.
Turkey, in a letter to the United Nations Security Council and UN chief Ban Ki-moon, said Monday its plane had gone down in international airspace.
"The aircraft did not display any hostile attitude or manoeuver and was flying with its identification systems open. The shooting came without any prior warning," said the letter which was obtained by AFP.
Turkey's UN ambassador Ertugrul Apakan said the incident was "a serious threat to peace and security in the region, in the context of the Syrian crisis" but did not call for the world body to act against Damascus.
Syria continued to insist that its downing of the Turkish F-4 Phantom jet on Friday had been justified.
"The Turkish warplane violated Syrian airspace, and in turn Syrian air defences fired back and the plane crashed inside Syrian territorial waters," Syrian foreign ministry spokesman Jihad al-Makdissi said Monday.
"What happened is a gross violation of Syrian sovereignty," he added.
The incident has reignited international concern over the Syria conflict.
European Union foreign ministers meeting in Luxembourg condemned the attack and announced new measures targeting Syrian government ministries and companies, including a bank and a television channel.
But a day ahead of a NATO meeting called at Turkey's request, the ministers also warned of the dangers of a military escalation in the crisis, and praised Turkey's "measured and responsible initial reaction."
NATO members Britain, France and the United States have all condemned Syria, with Britain saying Damascus should not be allowed to act with impunity.
German Foreign Minister Guido Westerwelle said it was "important that all forces understand that de-escalation is now decisive."
Turkey called Tuesday's emergency NATO meeting by invoking Article Four of the alliance's founding treaty, which covers threats to members' security.
"The facts in our possession show that our plane was hit by a heat-seeking guided laser missile," Turkey's Vice Prime Minister Bulent Arinc said.
The jet had been "intentionally shot down... in international airspace," he added.
"To target an aircraft in this fashion without any warning is a hostile act of the highest order," he said, adding that Ankara could soon announce a cut in power supplies to Syria.
Later Monday, Arinc accused Syria of having opened fire on a rescue plane searching for the jet's two pilots who are still missing.
"Everyone must know that this sort of behaviour will not go unpunished," he said.
In New York, the US ambassador to the United Nations gave a withering assessment of UN efforts to end the bloodshed inside Syria.
"The situation in Syria represents a colossal failure by the Security Council to protect civilians," 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 added.
She called for tougher sanctions against Syrian President President Bashar Al-Assad, a position backed by Britain.
Inside Syria, the killing continued.
The Syrian Observatory for Human Rights said 95 people including 61 civilians had died across the nation Monday, as the army pounded rebel strongholds and other towns and cities.
According to the Observatory, the conflict has cost more than 15,000 lives.
An AFP correspondent reported fierce clashes in the Homs province town of Qusayr between troops and rebel fighters followed by an army assault on the town during in which helicopter gunships were used.
And as pro-regime troops bombarded the central city of Homs, the rebel Free Syrian Army warned of an impending "massacre."
The opposition Syrian National Council called for international help "before it is too late."
But the Red Cross said that booby traps and the lack of a clear agreement from troops and rebels were blocking its efforts to evacuate the sick and wounded from Homs.
A Turkish diplomat told AFP that one Syrian general, two colonels and five other army officers including two majors, accompanied by 24 family members, had crossed into Turkey late on Sunday.
That brought to 13 the number of generals seeking refuge in the country since the revolt against the government of President Bashar Al-Assad erupted in mid-March 2011.