DIYARBAKIR, Turkey (Reuters) - A Turkish soldier was killed by an explosive device in southeastern Turkey, the first such death since Kurdish militants declared a ceasefire a year ago, security sources said on Thursday.

The Kurdistan Workers Party (PKK) called the truce in March 2013 to help bolster peace talks between the Turkish state and their jailed leader, Abdullah Ocalan.

The explosion occurred late on Wednesday when the soldier stepped on a device buried in the ground in a remote area in Sirnak province near the Iraqi border, the sources said.

It was not clear what caused the device to detonate, they said. In the past, PKK rebels have used remote-controlled devices to attack troops, or it may have been a land mine placed there previously.

The PKK took up arms in 1984 in a bid to carve out an ethnic homeland, but it has since scaled back its demands to greater political and cultural rights for Turkey's estimated 15 million Kurds.

Ocalan, a prisoner on an island near Istanbul, and Turkish authorities began negotiations in late 2012 in an effort to end a conflict that has killed more than 40,000 people, mainly Kurds, in three decades.

(Reporting by Seyhmus Cakan, writing by Ayla Jean Yackley; Editing by Gareth Jones)