* Deal follows upsurge in convoy attacks
* Violence at worst levels since 1990s
ANKARA, Dec 20 (Reuters) - Turkey signed an agreement on Thursday to transport troops to and from their military bases by air after a string of fatal attacks on road convoys prompted it to rethink security their arrangements.
The agreement, with state carrier Turkish Airlines , follows a dramatic increase in attacks on Turkish security forces in the past 18 months by Kurdistan Workers Party (PKK) militants, who have launched a series of fatal raids on troop convoys in the mainly Kurdish southeast.
Turkey's government and military began discussing alternative transport arrangements with the airline after a bomb attack on a security convoy in southeastern Bingol province which killed 10 people in September.
At the signing ceremony on Thursday, Turkey's defence minister said he envisaged some 250,000 soldiers and non-commissioned officers would be transported under the scheme. It will begin on Dec. 28.
Flights will be free of charge for troops with all costs met by the defence ministry. Other private airlines have also agreed to transport the soldiers, when needed, at a reduced cost and at no charge to the troops.
Soldiers will be flown to existing airports and transferred from there to outlying bases.
Military service is compulsory in Turkey, which has the second-largest army in NATO, with around half a million men.
Violence between the state and the PKK has reached levels not seen since the 1990s with at least 870 people killed between June 2011 and the end of last month, according to a tally by the think-tank International Crisis Group (ICG).
More than 40,000 people have been killed in the conflict between Turkey and the PKK since it took up arms against the state in 1984 with the aim of carving out an ethnic homeland in the southeast.
The PKK, designated a terrorist organisation by the United States and the European Union as well as by Turkey, has since softened its demands for greater autonomy for Turkey's Kurds. (Writing by Jonathon Burch; Editing by Andrew Osborn)