A female member of the outlawed Kurdistan Workers Party (PKK) was one of two suspected perpetrators of a car bombing that killed 37 people in the Turkish capital Ankara, security officials said on Monday.

Sunday's attack, tearing through a crowded transport hub a few hundred metres (yards) from the Justice and Interior Ministries, was the second such strike at the administrative heart of the city in under a month.

Evidence has been obtained that one of the bombers was a female member of the PKK who joined the militant group in 2013, the security officials told Reuters. She was born in 1992 and from the eastern Turkish city of Kars, they said.

Violence has spiralled in Turkey's predominantly Kurdish southeast since a 2-1/2 year ceasefire with the PKK collapsed in July. But the militants, who say they are fighting for Kurdish autonomy, have largely focused attacks on the security forces in southeastern towns, many of which have been under curfew.

Attacks in Ankara and in 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 the containment of violence across its borders in Syria and Iraq. President Tayyip Erdogan is also eager to dispel any notion he is struggling to maintain security.

"With the power of our state and wisdom of our people, we will dig up the roots of this terror network which targets our unity and peace," Foreign Minister Mevlut Cavusoglu said on Twitter. Turkish warplanes bombed camps belonging to the PKK in northern Iraq early on Monday, the army said. A round-the-clock curfew was also imposed in the southeastern town of Sirnak in order to conduct operations against Kurdish militants there, the provincial governor's office said.

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.

The government 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 sees the Syrian Kurds as an important ally in battling Islamic State.

BOMB PACKED WITH NAILS

A police source said hours after the explosion that there appeared to have been two attackers, a man and a woman, whose severed hand was found 300 metres from the blast site.The explosives were the same kind as those used in a Feb. 17 attack that killed 29 people, mostly soldiers, and the bomb had been packed with pellets and nails to cause maximum injury and damage, the source told Reuters.

The government has said it expects to officially identify the organisation behind the attack later on Monday. As part of a US-led coalition fighting Islamic State in neighbouring Syria and Iraq, Turkey faces multiple security threats.

Islamic State militants have been blamed for at least four bomb attacks on Turkey since June 2015, including a suicide bombing that killed 10 German tourists in the historic heart of Istanbul in January. Local jihadist groups and leftist radicals have also staged attacks in Turkey in the past.

There was little immediate reaction on 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.

"It is clear that Turkey's political risk profile is rising gradually and the country is not yet safe for long-term investors," Atilla Yesilada of Istanbul-based consultancy Global Source Partners said in a note to clients.

In its armed campaign in Turkey, 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 previous car bombing, just a few blocks away, on Feb. 17. TAK says it has split from the PKK, although experts who study Kurdish militants say the two organisations are affiliated.