Turkey's state-run news agency says 33 more members of the Syrian military have defected to Turkey with their families at a time of heightened tensions between the two countries over Syria's downing of a Turkish plane.

The Anadolu agency said on Monday that the group, which includes a general and two colonels, crossed into Turkey overnight and that they were being hosted at a refugee camp near the border.

The defection brought to 13 the number of generals seeking refuge in Turkey since the revolt against Syrian President Bashar al-Assad's regime erupted 16 months ago.

Thousands of soldiers have abandoned the army, but most are low-level conscripts. The Free Syrian Army, the loosely-linked group of rebel forces, is made up largely of defectors.

Turkey, once a close ally of Syria, called an extraordinary NATO meeting for Tuesday after accusing Damascus of shooting down one of its military planes in international airspace.

The Syrian government has disputed that claim; Jihad Makdissi, a spokesman for the foreign ministry, said at a press conference that "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," he said. "If the goal of the [NATO] meeting is to calm the situation and promote stability, we wish it success... 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."

Fresh sanctions

Uri Rosenthal, the Dutch foreign minister, on Monday said that the European Union will condemn Syria's downing of a Turkish jet, but will not support military intervention in Syria. "What happened is to be considered very seriously [but] we do not go for any interventions," he said.

The EU is planning to add another Syrian official and six firms and government institutions to a sanctions list which already includes more than 120 individuals and nearly 50 entities, a spokesman said. The asset freeze and travel ban will be the 16th round of restrictive EU measures imposed on Assad's government. No details were immediately available on the identities of those targeted.

The sanctions also include a specific ban on insuring items embargoed for delivery to Syria, including arms shipments.

In a seperate development, Syrian troops pounded the central city of Homs on Monday amid warnings from the rebel Free Syrian Army (FSA) of an impending "massacre" there. The Syrian National Council (SNC), the main political opposition bloc, issued a distress plea from residents, urging international help "before it is too late."

Activists reported shelling of the neighbourhoods of Jouret al-Shayyah and al-Hamidiyeh using rockets and artillery.

The FSA said "Arab and Islamic countries, friendly nations and concerned international organisations" bore responsibility for what happens.

"The brave city of Homs faces the strongest and most violent bombardment of rockets, artillery and tanks," its Supreme Military Council said in a statement. "The regime is sending reinforcements estimated at 100 tanks in the direction of Homs... which clearly demonstrates its intention to commit the greatest massacre in history."