BEIRUT — Syria shot down a warplane from Turkey on Friday that it said had violated its airspace, an event that illustrated the potential for the Syrian conflict to spill across its borders and risked a further deterioration in relations between neighbors that once were close allies.

In a terse statement after midnight, the office of Turkish Prime Minister Recep Tayyip Erdogan said Syria had downed a Turkish plane that disappeared about midday off the Syrian coast. The fate of the two pilots was unknown.

The official Syrian Arab News Agency, known as SANA, said the aircraft, flying low and fast, violated Syrian airspace over the Mediterranean Sea and was shot down by ground fire. It said the plane was hit about half a mile from the Syrian coast and crashed into the sea.

The incident appears to be another example of how the Syria conflict has elevated regional tensions. Syria sits in the heart of the volatile Middle East and its instability has already had ramifications in Turkey, Lebanon and elsewhere.

Erdogan’s office said Turkey would act “decisively” once all the details of the incident emerged. It did not specify what action it might take, or whether it would act alone or in concert with its NATO allies.

Erdogan earlier told reporters that Turkish and Syrian authorities had launched a search for the pilots focused on a point in the Mediterranean about eight miles off the Syrian port of Latakia.

Turkey has joined the United States and other allies in calling for Syrian President Bashar Assad to step down in the face of a rebellion that is now more than a year old.

The Turkish-Syrian border, which is more than 500 miles long, has become increasingly tense, as tens of thousands of refugees have fled the fighting in Syria and taken up residence in camps on the Turkish side.

Several cross-border shooting incidents have been reported in recent months, and Syria has called the border zone an entry point for weapons destined for Syrian rebels. Turkey has denied reports that it has sold arms or facilitated arms transfers to Syrian insurgents.

Turkey has become a haven for Syrian rebels seeking to overthrow Assad. A major Syrian opposition umbrella group, the Syrian National Council, has its base in Istanbul. Still, the two sides have generally sought to downplay border tensions.

The downing of the plane “is one of those spillover effects that could make this conflict much bigger,” Andrew J. Tabler, a Syria expert at the Washington Institute for Near East Policy, told the BBC.

In Geneva on Friday, United Nations special envoy Kofi Annan called for Iran to be included in the next efforts to bring peace to Syria, countering U.S. objections toTehran’s involvement by saying all nations with influence on the warring parties should play a role.

Annan’s six-point peace plan for ending the armed conflict has been widely violated by the Syrian government and by rebel forces.

In Syria, a SANA report charged that “armed terrorist groups” — the government’s label for armed rebels — had massacred more than 25 people outside the city of Aleppo.

A British-based opposition group, the Syrian Observatory for Human Rights, apparently describing the same incident, said at least 26 people “thought to be pro-government thugs” were shot and killed near Aleppo.

patrick.mcdonnell@latimes.com

Marrouch is a special correspondent. Times staff writer Carol J. Williams in Los Angeles contributed to this report.