Two of the suspects had apparently received training in bomb-making in camps belonging to the outlawed Kurdistan Workers' Party (PKK), the agency said.

Three bomb attacks shook Istanbul in late March and early April, killing one person and inuring more than 40, while unprecedented Kurdish riots in towns in the southeast of the country claimed 12 lives.

All were claimed by the Kurdistan Freedom Falcons (TAK), which Turkish officials say is a group the PKK uses as cover for attacks on urban civilian targets. The PKK says TAK is a breakaway group over which it has no control.

Clashes between the PKK and the army have increased markedly since the riots and the army has amassed troops in the mainly Kurdish-populated southeast to crackdown on rebels thought to be infiltrating Turkey from bases in northern Iraq.

More than 37,000 people have been killed since 1984 when the PKK, branded a terrorist group by Turkey, the European Union and the United States, began fighting for self-rule in the southeast.