Turkey's rebel Kurdistan Workers Party (PKK) condemned a bomb attack that killed three people in Ankara and was claimed by a radical Kurdish group, a pro-Kurdish news agency said Saturday.

"The leadership of the PKK said in a statement that this type of attack was reprehensible and harmful to the legitimate demands of the Kurdish people," Firat News reported.

In an email sent to Firat News on Thursday, the Kurdistan Freedom Falcons (TAK) said it had carried out Tuesday's attack and warned: "It is only a beginning."

The group threatened to spread violence to urban areas, after a wave of deadly rebel attacks on army and police units in the southeast.

"Everywhere is a target," it said.

The bomb, which went off outside the administration offices in the Turkish capital, injured at least 15 people.

Turkish officials say TAK is a front used by the PKK, listed as a terrorist group by Ankara and much of the international community.

But the PKK says TAK is a splinter group outside its control.

Firat News said the PKK also expressed regret for the death of four civilians in an attack it carried out on Tuesday at Siirt in the southeast of the country.

It said the leadership had called on all guerrilla units to be more careful in planning attacks and warned that commanders would be held responsible for such mistakes.

Kurdish rebels fighting for autonomy in southeastern Turkey have recently escalated their attacks on Turkish targets.

Three security officers were killed in two separate attacks in east and southeast Turkey, on Wednesday and Thursday.

Local security sources said Saturday that another policeman had died of wounds sustained in Thursday's attack at Dyarbakir, the main city of the Kurdish-majority southeast.

The successive assaults came days after Turkey's government threatened to launch a ground assault on PKK camps across the border in northern Iraq.

The Turkish air force has repeatedly bombed PKK targets since August 17, the latest raid being on Friday.

The PKK took up arms in Kurdish-majority southeast Turkey in 1984, sparking a conflict that has claimed about 45,000 lives.