The suspect, who allegedly purchased the car used in Thursday's bombing which killed six people, was detained late Monday along with six others, Interior Minister Besir Atalay said.

Police seized weapons and explosives at their homes, he said.

"Seven people are in custody. One of them is the person who bought the car several days before (the attack) and the others are people linked to him," Atalay told the semi-official Anatolia news agency.

Quoting police officials in Diyarbakir, Anatolia identified the suspect who bought the car and detonated the bomb as 23-year-old E.P., a militant trained in explosives in Kurdistan Workers' Party (PKK) camps in northern Iraq.

Atalay said one of the suspects had returned to Turkey from northern Iraq last year.

The car bomb was set off by remote control Thursday in central Diyarbakir, the largest city in Kurdish-majority southeast Turkey, as an army vehicle with some 50 soldiers on board was passing by.

The death toll climbed to six Tuesday as a high school student died from his injuries, a hospital spokesman said.

Four other dead were also teenagers attending classes at a nearby private school; nearly 70 others were injured, about 30 of them soldiers.

Turkish officials had already blamed the bombing on the PKK, which has been fighting for Kurdish self-rule in southeast Turkey since 1984.

The PKK apologised for the attack Tuesday and put the blame on militants acting on their own, according to the Firat news agency, considered a rebel mouthpiece.

"This attack was not planned centrally by our movement... We regret the loss of civilian life and apologise to our people," Firat quoted Bozan Tekin, a senior PKK member, as saying.

"According to our investigation, it was an act by independent, local (PKK) units... It targeted a vehicle carrying military officers," he said.

Police say the bomb consisted of 40 kilogrammes (88 pounds) of plastic explosives of a type the PKK has frequently used.

Turkish army chief Yasar Buyukanit last week described the blast as a sign of "panic" in PKK ranks following Turkish air strikes in December on rebel bases in northern Iraq, against which the PKK had threatened to retaliate.

The military has confirmed three air raids conducted with US intelligence assistance against the PKK in Iraq since December 16 in which it said at least 150 rebels were killed and more than 200 PKK positions destroyed.

The PKK has been blamed for several recent bomb attacks, including one in June near a bus stop in central Diyarbakir that wounded seven.

In 2006, 10 people, including seven children, were killed and 14 injured at a crowded city park here in a blast officials also blamed on the PKK.

More than 37,000 people have been killed since the PKK, listed as a terrorist group by Ankara and much of the international community, took up arms for Kurdish self-rule in the southeast in 1984.