(Adds President Gul quote, fresh army quotes, background)

By Ferit Demir

TUNCELI, Turkey, Oct 7 (Reuters) - Kurdish rebels shot dead 13 Turkish soldiers on Sunday, the worst such incident in years and likely to put more pressure on the government to authorise a cross-border military strike against Kurdish bases in Iraq.

The soldiers were killed after the army -- which has boosted its troops in the southeast and introduced security zones limiting access for civilians in the region -- killed a PKK suspect earlier in the day in fighting in Sirnak province.

"Thirteen members of our armed forces were killed in an attack ... carried out by terrorists on one of our units serving in the Sirnak region," a General Staff statement said.

"The terrorists were pursued under fire by our units to escape routes out of the country to be stopped from leaving," it said in an apparent reference to Iraq, which borders Sirnak province.

Some 3,000 PKK fighters are based in northern Iraq and launch attacks on security and civilian targets in Turkish territory. Thousands of PKK rebels are also believed to be inside Turkey, which is seeking European Union membership.

The Turkish armed forces favour a cross-border operation, but Washington fears such a move could destabilise the only relatively stable part of Iraq. Turkey is a key NATO ally.

The ruling AK Party government has been reluctant to push for a cross-border operation, seeking instead to pressure Washington and Baghdad to act.

PRESSURE MOUNTS

"Those who create, feed and support terrorism should know that no force can stand against the determination of the Republic of Turkey to protect its inseparable integrity," the state Anatolian news agency quoted President Abdullah Gul as saying.

Last month Ankara and Baghdad signed an agreement to help clamp down on PKK militants on Iraqi territory, but it did not give Turkey permission to follow rebels into Iraq.

Turkey has criticised both Iraqi and U.S. authorities for failing to control the rebels.

There have been pockets of intense fighting in the southeast between the Kurdish rebels and the army before winter sets in.

Helicopters hit rebel positions after the soldiers were killed, security officials, who declined to be identified, said.

Gul and Prime Minister Tayyip Erdogan offered condolences to armed forces chief General Yasar Buyukanit, Anatolian reported.

In a separate incident, a bomb near a ferry terminal on the outskirts of Istanbul wounded four people, including a policeman. Kurdish rebels, as well as al Qaeda and left-wing militants, have been blamed for such attacks against civilians and the authorities in the past.

On Sept. 29, PKK members stopped a minibus in Sirnak province and killed 12 passengers. A Turkish soldier was killed in the region on Oct. 6.

The PKK took up arms against Turkey in 1984 with the aim of creating an ethnic homeland in the southeast. More than 30,000 people have been killed in the conflict.

The fighting dwindled after the capture of PKK leader Abdullah Ocalan in 1999, but clashes have resumed in recent years. The PKK is considered a terrorist organisation by Turkey, the United States and the European Union.

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