The Syrian civil war threatened to spread beyond its borders last night as the Turkish army launched artillery attacks against Syria in response to deadly shelling that killed five people in south-east Turkey.

Nato ambassadors held an emergency meeting in Brussels last night to discuss the incident, the first overt military intervention in the Syrian conflict by an outside power. The alliance condemned Syria, and called for an end to “flagrant violations of international law”.

The Turkish artillery strikes came in retaliation to shelling – believed to be from Syrian government forces fighting rebels near the border – of the village of Akcakal, in Turkey’s south-east. Turkish officials said five people were killed in the attack. Syria said it was investigating the source of the shell and extended condolences to the Turkish people.

A statement from the office of Turkey’s Prime Minister, Recep Tayyip Erdogan, said: “Our armed forces in the border region responded immediately to this abominable attack in line with their rules of engagement; targets were struck through artillery fire against places in Syria identified by radar.

“Turkey will never leave unanswered such kinds of provocation by the Syrian regime against our national security,” it added.

The UN Secretary-General Ban Ki-moon urged Damascus to respect the territorial sovereignty of its neighbours.

"Today's incidents... again demonstrated how Syria's conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbours,” his spokesman said in a statement.

US Secretary of State Hillary Clinton had earlier expressed outrage over the incident, and said Washington would discuss with Ankara on what the next steps should be.

"We are outraged that the Syrians have been shooting across the border. We are very regretful about the loss of life that has occurred on the Turkish side," she said.