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Turkish soldiers, air force bombers and helicopter gunships conducted a major offensive in southeastern Turkey on Friday after Kurdish legislators declared autonomy in the region and more than a dozen soldiers were killed there by Kurdish rebels.

After Friday prayers in this mostly Muslim nation, Prime Minister Recep Tayyip Erdogan said violence by the Kurdish rebels in the Diyarbakir area will achieve nothing.

"What they did is never going to drag us to the table," Erdogan said of the Kurdish guerrillas. "If they want to make peace, there is only one thing to do: the terrorist organization must lay down arms," he said in nationally televised comments in Istanbul.

In a rare show of unity, Erdogan's ruling party and the opposition issued a joint parliamentary declaration denouncing the violence and vowing solidarity against "terrorism and separatist attempts."

On Thursday, Kurdish guerrillas attacked Turkish forces in Diyarbakir, leaving 13 soldiers and seven rebels dead. It was the deadliest violence involving the Kurdish guerrillas in three years. That clash and the autonomy declaration by the regional Kurdish legislators also sparked anti-Kurdish protests across Turkey, including a firebomb attack on a closed office of the Kurdish political party on Thursday night in Ankara, the capital.

The rebel attack, and the autonomy vote, occurred hours after lawmakers from the country's Kurdish party and the government failed to reach an agreement to end a boycott of Turkey's Parliament in Ankara by Kurdish legislators.

Kurdish lawmakers have said they will not take their oath of office until five pro-Kurdish legislators who are charged with ties to Kurdish rebels are released from jail and another Kurdish politician, Hatip Dicle — whose election to Parliament was canceled due to a conviction for ties to the rebels — is allowed to work in Parliament.

The military's offensive involves hundreds of elite soldiers sent to remote areas of southeastern Turkey where Kurdish rebels are believed to be hiding. The area is near northern Iraq, where Kurdish rebels have long been based. Turkey's military declined to comment when asked if the offensive has caused casualties.

Kurdish politicians have long pushed for greater cultural and political rights for Kurds, who make up around 20 percent of Turkey's 74 million people. Since Kurdish rebels took up arms in 1984 to seek autonomy in their southeastern region, the conflict has killed nearly 40,000 people.

After an umbrella group that includes Turkey's Kurdish party proclaimed Kurdish autonomy in Diyarbakir on Thursday, the prosecutor's office the region's largest city said it was examining the declaration, which the government sees as a threat to national unity. Prosecutors are expected to press criminal charges against dozens of Kurdish lawmakers, politicians and activists after the probe.

Meanwhile, NATO Secretary-General Anders Fogh Rasmussen denounced Thursday's rebel attack on Turkish soldiers.

"I strongly condemn the terrorist attack in Diyarbakir province," Rasmussen said in a statement on Friday. "Such heinous attacks have no justification. I express my heartfelt condolences to the families of those who were killed. NATO allies stand in full solidarity against the scourge of terrorism."

Dutch legislator Ria Oomen-Ruijten, a member of the European Parliament, also condemned the attack and urged Kurdish lawmakers to distance themselves from the rebels who are regarded as a terrorist group by the West.

"I urge the newly elected (Kurdish) members of Parliament to distance themselves from this unacceptable violent attack and call upon them to make a fresh start in the peace settlement," she said. "The only way forward is through political dialogue and concrete initiatives for reconciliation."

In Turkey on Friday, top generals and senior Cabinet members stood by coffins of the dead soldiers draped in the red and white Turkish flag at a solemn funeral service at a military base in Diyarbakir and prayed for them.

Thursday's military casualties were the highest since the rebels of the Kurdistan Workers' Party killed 17 soldiers in an October 2008 attack on a military unit on the Iraqi border.

The rebels also killed a dozen soldiers in an ambush along the Iraqi border in October 2007, an attack that triggered a weeklong air-and-ground Turkish offensive in early 2008 against Kurdish rebel bases in northern Iraq.

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Associated Press Writers Suzan Fraser and Ozgur Akman contributed to this report.