"Several" Syrian soldiers died when Turkish troops shelled their military post in response to earlier cross-border mortar fire which smashed into a Turkish town killing five people, a watchdog said on Thursday.

The Syrian Observatory for Human Rights said overnight shelling across the frontier by Turkish troops hit a Syrian army post near the border town of Tel Abyad. It gave no exact figure for the number of soldiers killed.

Turkey has demanded that the UN Security Council take action against Damascus after Wednesday's incident, in which mortar fire from Syria killed five Turkish civilians, including a mother and her three children, in the Turkish town of Akcakale.

Turkish artillery fired retaliatory salvos back at Syrian targets during the night, marking a sharp escalation in the crisis between the neighbours.

The artillery fire resumed again on Thursday morning, a Turkish security source told AFP on condition of anonymity.

The Wednesday cross-border shelling was the first to kill Turkish civilians, making this the most serious clash since June, when Syria shot down a Turkish fighter jet and killed both its crewmen.

NATO backs Turkey, calls for Syria to respect law
Brussels (AFP) Oct 03, 2012 - NATO in an emergency meeting Wednesday backed Turkey and called on Syria to respect international law after a Syrian shell hit a border town, killing five Turkish nationals and sparking retaliatory strikes.

"The Alliance continues to stand by Turkey and demands the immediate cessation of such aggressive acts against an Ally and urges the Syrian regime to end flagrant violations of international law," a NATO statement said after the meeting was called at member Ankara's request.

The alliance said Syria's "recent aggressive acts at NATO's southeastern border" were a "flagrant breach of international law and a clear and present danger" to an ally.

"The most recent shelling... which caused the death of five Turkish citizens and injured many, constitutes a cause of greatest concern and is strongly condemned by all Allies," it added.

Earlier, NATO head Anders Fogh Rasmussen had spoken to Turkish Foreign Minister Ahmed Davutoglu to express "his strong condemnation of the incident in Akcakale" town, a spokeswoman said.

Turkey shelled Syria in retaliation for what it termed an attack "too far."

"Our armed forces in the border region immediately retaliated against this heinous attack... by shelling targets spotted by radar," Turkey's prime minister's office said in a statement earlier.

"Turkey will never leave unanswered such provocations by the Syrian regime targeting our national security, in line with engagement rules and international law."

The incident is the most serious between the two neighbours since June when Syria shot down a Turkish fighter plane with the loss of its two-man crew and Ankara took the matter to NATO.