Syria insisted on Monday that a Turkish warplane shot down by its forces violated its airspace, but Ankara hit back saying it was a "hostile act of the highest order" ahead of NATO emergency talks.
The incident has reignited concern over the Syria conflict, with the European Union condemning the Damascus regime and slapping new sanctions against it while also warning of the dangers of military escalation.
Turkey charged that Syria "intentionally" shot down the F-4 Phantom, and Vice Prime Minister Bulent Arinc told the cabinet on Monday "a heat-seeking guided missile" struck the jet.
Arinc also warned that Turkey could cut electric power supplies to energy-poor Syria in retaliation.
As tensions simmered, a Syrian general was among a new group of officers and soldiers to defect and join the growing rebel ranks in Turkey, media reports said.
On the ground, 53 people, most of them civilians, were reported killed nationwide, amid opposition claims the regime was preparing for a new "massacre" in the besieged flashpoint city of Homs.
The Red Cross said 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.
NATO member states are to hold a crucial meeting on Tuesday to discuss Friday's downing of the Turkish jet, as Ankara and Damascus traded blame for what happened.
Turkey says the jet was fired at over international waters, not inside Syrian airspace as Damascus maintains, and is urging ministers to honour the collective defence rights of a fellow NATO member.
Syrian foreign ministry spokesman Jihad al-Makdissi told a news conference the plane had violated Syrian airspace.
"The Turkish warplane violated Syrian airspace, and in turn Syrian air defences fired back and the plane crashed inside Syrian territorial waters.
"What happened is a gross violation of Syrian sovereignty.
"If the goal of the (NATO) meeting is to calm the situation and promote stability, we wish it success," Makdissi said.
But "if the goal of the meeting is aggression, we say that Syrian airspace, territory and waters are sacred for the Syrian army, just as Turkish airspace, territory and waters are sacred for the Turkish army."
Turkey has called Tuesday's emergency NATO meeting, 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," Arinc said, adding that the jet was "intentionally shot down... in international airspace."
"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 much-needed power supplies to Syria.
The jet had been on an unarmed training mission, and both crew are still missing.
The downing of the Turkish jet triggered a chorus of international condemnation as well as appeals for restraint to prevent a military escalation of the Syria conflict which is now in its 16th month.
EU foreign ministers meeting in Luxembourg sought to up the pressure on Syria, imposing new sanctions targeting government ministries and companies, including a bank and a television channel.
But on the eve of the NATO meeting, many also cautioned against military intervention.
In a joint statement condemning the incident, the ministers praised Turkey's "measured and responsible initial reaction," and said the matter needed to be investigated "thoroughly and urgently."
It was "important that all forces understand that de-escalation is now decisive," German Foreign Minister Guido Westerwelle said.
NATO members Britain, France and the United States have all condemned Syria, with Britain saying Damascus should not be allowed to act with impunity.
French Foreign Minister Laurent Fabius called the shooting down "completely unacceptable," while US Secretary of State Hillary Clinton condemned a "brazen and unacceptable act" and promised US assistance in the investigation.
A Turkish diplomat, meanwhile, told AFP that one Syrian general, two colonels and five other army officers including two majors, accompanied by 24 family members, crossed into Turkey late on Sunday.
Turkey's Anatolia news agency had initially put the number at 33 defectors -- the general, two colonels and 30 soldiers.
The latest defections 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.
The violence has killed more than 15,000 people, the Syrian Observatory for Human Rights says.
On Monday, the Britain-based watchdog reported at least 53 people -- most of them civilians, as well as 15 army troops -- were killed across Syria, as the army pounded rebel strongholds and other towns and cities.
Troops pounded the central city of Homs, amid rebel Free Syrian Army warnings of an impending "massacre," and the opposition Syrian National Council urging international help "before it is too late."
And 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 which helicopter gunships were used.