The court ordered that three of the suspects be jailed and the remaining six be released, but all pending trial, the report said.

Details of the charges will become known when prosecutors draw up their indictment over Tuesday's attack, which killed four soldiers and the teenage daughter of an officer and left 14 people wounded.

Ten other people detained by police as part of the probe were released without charge after questioning, Anatolia said.

Radical Kurdish militants claimed responsibility for the remote-control roadside bomb that blew up the bus in the suburb of Halkali.

Police are still looking for the assailant who detonated the device, Istanbul Governor Huseyin Avni Mutlu said earlier Thursday.

Some of the suspects had already been arrested Monday as part of a probe into the June 8 bombing of a bus carrying policemen to work in the same district, which injured 15 people, Mutlu explained.

"The 19 suspects were handed over to the judicial authorities along with evidence that they were involved in the preparation stages of both attacks," he said.

Police believe the suspects were linked to "the terrorist organisation," he said, using the authorities' preferred term for the armed separatist Kurdistan Workers' Party (PKK), listed as a terrorist group by Ankara and much of the international community.

Ten other people remain in police custody for questioning, he added.

Tuesday's attack and the June 8 blast were both claimed by the Kurdistan Freedom Falcons (TAK), an obscure radical group loyal to jailed PKK leader Abdullah Ocalan, jailed for life in 1999.

The Turkish authorities say TAK is a front used by the PKK, especially when attacks claim civilian casualties.

The PKK has said TAK is a splinter group outside its control.

The rebels have dramatically stepped up attacks since Ocalan said through his lawyers last month he was abandoning efforts to seek dialogue with Ankara to peacefully end the 26-year Kurdish conflict.

The PKK threatened to spread the violence to urban areas after killing 12 soldiers in attacks in remote regions in the Kurdish-majority southeast at the weekend.

The group took up arms for self-rule in the southeast in 1984, sparking a conflict that has claimed some 45,000 lives.