Syria says it has shot down a Turkish air force jet that had flown into Syrian airspace.
Syrian officials said the low-flying warplane was shot down over the Mediterranean Sea Friday.
Turkish political and military leaders held a two-hour security meeting in Ankara to discuss the incident.
After the meeting, Prime Minister Recep Tayyip Erdogan's office said Turkey will decide on how to respond to the incident but did not elaborate.
"We have no word on the pilots, but Turkish helicopters as well as four assault boats are searching for the pilots along with Syrian ships. We will see the outcome of this joint search operation," said Erdogan.
Syrian forces are helping the Turks search for the two missing Turkish pilots.
Turkey has joined nations such as the U.S. in saying that Syrian President Bashar al-Assad should step down because of the uprising in his country.
Meanwhile, U.N. Envoy for Syria Kofi Annan told reporters in Geneva Friday that "countries of influence" need to persuade both sides in Syria to "stop the killing and start the talking." He said it is time for countries to "raise the level of pressure" on both Syria's government and the opposition to end violence in the country.
Mr. Annan said Iran should be "part of the solution" for bringing peace to Syria. Russia has pushed for Iran's involvement in resolving the conflict, while the U.S. has said Iran should not be involved.
The U.N. special envoy's comments came after Syrian activists said 26 men reported to be pro-government "shabiha" militia fighters were killed in an ambush in the northern Aleppo province.
Rami Abdelrahman, the head of the Britain-based Syrian Observatory for Human Rights, told VOA on Friday that he received a video showing the bloodied bodies of the men piled on the side of a road.
"Over 26 people following the Syrian regime were killed west of Aleppo. This morning the people there said these people are from shabiha. We cannot confirm this, but we have received a video with over 26 bodies," said Abdelrahman.
In an apparent reference to the same incident, Syrian state media reported Friday that at least 25 people were killed by "armed terrorists" in the same region outside Aleppo.
The Observatory said it could not confirm whether the victims were civilians, as Syrian state media reported, or shabiha militants, who have been blamed for a series of deadly attacks on civilians in recent months.
The group also reported fresh clashes Friday between government troops and rebels in the flashpoint city of Homs, where hundreds of civilians are believed to be trapped and unable to find shelter.
The International Committee of the Red Cross aborted its attempt to evacuate civilians from Homs Thursday after encountering close gunfire. The ICRC said it will attempt to go back to the area as soon as conditions permit.
The United Nations says up to 1.5 million Syrians are now in need of humanitarian assistance as it warns of a deteriorating situation in Syria. The figure reported by the U.N. Office for the Coordination of Humanitarian Affairs is up from an earlier estimate of 1 million.
Some information for this report was provided by AP, AFP and Reuters.