(udpates with details, background)
ANKARA, March 20 (Reuters) - Turkish Prime Minister Tayyip Erdogan described the gunning down of three members of the security forces in the southern province of Nigde on Thursday as a "nefarious terrorist attack" but did not say who was responsible.
Paramilitary and regular police officers were manning a checkpoint when the attackers opened fire from a truck, Turkey's Dogan News Agency (DHA) said. The driver of the truck, which was stolen, was also killed in the exchange of fire.
Speaking at a rally before March 30 local elections in the northwestern town of Sakarya, Erdogan confirmed the attack.
"A nefarious terrorist attack has been carried out against our gendarmarie and police in the Ulukisla district of Nigde," Erdogan said.
Two of the assailants were captured alive and police were searching for a third, he added.
Five other members of the security forces were wounded, local news agencies said.
Southeastern Turkey has seen similar attacks by Kurdish militants in the past but the region has been relatively calm since a ceasefire was announced last year as part of a peace process with the government.
Isolated clashes have occasionally threatened the ceasefire, which is still holding as Turkey and the Kurdistan Workers Party's (PKK) jailed leader, Abdullah Ocalan, negotiate a peace process to end a conflict that has killed more than 40,000 people over the past decade.
The aim of the assailants were not immediately clear. A statement from Turkish deputy prime minister Besir Atalay raised the possibility that the attack could have been carried out by Syrians.
"In the information I received, there is a note about Syria. It is very grave. They may be aiming to stir up the election atmosphere" Atalay was quoted as saying by Hurriyet newspaper.
Turkey has seen Syria's three-year-long conflict spilling into its borders occasionally, with mortar shells exploding on its soil, causing civilian deaths.
Turkey, which share a 900-km border with Syrian, has been among the fiercest opponents of President Bashar al-Assad and has been housing more than 700,000 Syrian refugees.
About half a million refugees live outside the camps, paving the way for increasingly disgruntled local communities who complain about a rise in robberies and crime.
(Reporting by Humeyra Pamuk; Writing by Nick Tattersall; Editing by Angus MacSwan)