By Orhan Coskun

ANKARA (Reuters) - Turkish warplanes struck Kurdish militant camps in northern Iraq on Monday, a day after 37 people were killed in an Ankara car bombing that security officials said involved two fighters - one female - from the Kurdistan Workers Party (PKK).

Sunday's attack, tearing through a crowded transport hub a few hundred meters from the Justice and Interior Ministries, was the second such strike at the administrative heart of the Turkish capital in under a month.

No one has claimed responsibility for the latest attack. However, security officials told Reuters a female member of the outlawed PKK, which has fought a three-decade insurgency for Kurdish autonomy in Turkey's southeast, was one of the suspected perpetrators. A police source said her severed hand had been found 300 meters from the blast site.

Evidence had been obtained that suggested she was born in 1992, was from the eastern city of Kars near the Armenian border, and had joined the militant group in 2013, they said. The second suspect was a male Turkish citizen also with PKK links, a security official said.

Deputy Prime Minister Numan Kurtulmus said a woman was "definitely" one of the suicide bombers and the second bomber was male, although his identity has not yet been confirmed. At least 11 people have been detained and 10 more were sought in connection with the attack, he said.

Violence has increased in Turkey's predominantly Kurdish southeast since a 2-1/2 year ceasefire with the PKK collapsed in July. The militants have so far largely focused their strikes on security forces in southeastern towns, many of which have been under curfew.

But attacks in Ankara and Istanbul over the last year, and the activity of Islamic State as well as Kurdish fighters, have raised concerns among NATO allies who see Turkey's stability as vital to containing violence in neighboring Syria and Iraq. President Tayyip Erdogan is also eager to dispel any notion he is struggling to maintain security.

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The Turkish military said 11 warplanes carried out air strikes on 18 targets in northern Iraq early on Monday, including ammunition depots and shelters. The PKK has its bases in the mountains of northern Iraq, controlling operations across the frontier in Turkey.

The strikes were launched after the perpetrators of the attack were determined, Prime Minister Ahmet Davutoglu said, adding the government had obtained "almost certain" findings that pointed to the PKK.

A round-the-clock curfew was declared in three southeastern towns in order to conduct operations against Kurdish militants, local officials said. Many local people fled the towns in anticipation of the operations.

Authorities detained 15 people in Istanbul in operations against the PKK and 50 more elsewhere in the country, CNN Turk reported.

Victims of Sunday's attack included the father of Umut Bulut, a footballer who played for Turkey and Galatasaray, the Istanbul club said on its website.

WAR IN SYRIA

Turkey's government sees the unrest in its southeast as closely tied to the war in Syria, where a Kurdish militia has seized territory along the Turkish border as it battles Islamic State militants and rebels fighting President Bashar al-Assad.

Ankara fears those gains are stoking Kurdish separatist ambitions at home and says Syrian Kurdish fighters share deep ideological and operational ties with the PKK.

They also complicate relations with the United States which, while deeming the PKK to be a terrorist group, sees the Syrian Kurds as an important ally in battling Islamic State. Such is the complexity and sensitivity of alliances in the region.

The attack is the third in five months to hit Ankara.

The explosives were the same kind as those used in a Feb. 17 attack in the city that killed 29 people, mostly soldiers, and the bomb had been packed with pellets and nails to cause maximum damage, a security official told Reuters.

More than 100 people were killed in a double suicide bombing in Ankara in October that has been blamed on Islamic State.

Turkey is part of the U.S.-led coalition fighting Islamic State in Syria and Iraq. The militant group has been blamed for at least four bomb attacks on Turkey since June 2015, including the killing of 10 German tourists in Istanbul in January. Local jihadist groups and leftist radicals have also staged attacks.

'WHO IS RESPONSIBLE?'

The main opposition CHP party criticized Interior Minister Efkan Ala, suggesting he had not done enough to ensure security.

"If a minister is not doing his duty, that person should leave their post," CHP leader Kemal Kilicdaroglu said. "Hundreds of our people have lost their lives because of terror. Who is responsible for this?"

Several hundred people - some chanting "thief Tayyip", a rallying cry against Erdogan used in previous anti-government protests - gathered near Istanbul's Taksim square to express anger over the attack, before police dispersed them using water cannon and tear gas.

The head of the pro-Kurdish Peoples' Democratic Party (HDP), Selahattin Demirtas, condemned the bombing as a "terrorist attack that directly targets civilians" and called on the government to give an account of what happened.

Erdogan has previously accused the HDP of being an extension of the PKK, calling for legal action against MPs.

On Monday he said it was necessary to broaden the definition of "terrorist" to include those who support terrorism, listing examples such as MPs, academics, journalists or civil society leaders.

Dozens of academics were detained in January accused of terrorist propaganda after they criticized military action in the southeast and urged an end to curfews.

There was little immediate reaction in financial markets, with the lira only slightly weaker against the dollar. But analysts said the deteriorating security situation was a concern for a country heavily dependent on tourism.

In its armed campaign, the PKK has historically struck directly at the security forces and says it does not target civilians. A direct claim of responsibility for Sunday's bombing would indicate a major tactical shift.

The Kurdistan Freedom Hawks (TAK) claimed responsibility for the February bombing. TAK says it has split from the PKK, although experts say the two are affiliated.

(Additional reporting by Asli Kandemir and Ayla Jean Yackley in Istanbul and Ece Toksabay and Ercan Gurses in Ankara; Writing by Daren Butler and David Dolan; Editing by Janet Lawrence)