A car bomb tore through a crowded transport hub in Ankara on March 13, killing dozens and injuring scores more. (Reuters)

Turkey targeted Kurdish militants with airstrikes on their strongholds in Iraq on Monday as officials claimed “almost certain” links between the group and a suicide car bombing that killed at least 37 people in the Turkish capital.

Sunday’s blast — less than a month after a similar attack in the capital, Ankara — sharply raised concern that Turkey’s long war with Kurdish separatists could be spreading from Kurdish regions in the country’s southeast to major urban centers.

There was no assertion of responsibility for the attack, but Turkish Prime Minister Ahmet Davutoglu said “very serious and almost certain” findings point to the Kurdistan Workers’ Party, or PKK, Turkey’s main militant faction.

The group has waged a violent campaign since the 1980s — including a brief lull — in its quest for greater autonomy in Turkey’s Kurdish heartland. It also maintains bases over the border in northern Iraq.

Battles between Kurdish militants and Turkish forces have escalated recently, adding yet another front in a region already in deep turmoil over the Syrian civil war and the Western-led campaign against the Islamic State militant group.

Davutoglu said 11 people were detained in Sunday’s blast — which occurred about 200 yards from his office — but authorities gave no details about their background or possible affiliations.

A woman was “definitely” one of the suicide attackers, Deputy Prime Minister Numan Kurtulmus said. Authorities suspect that the bombing was carried out by two people, and Turkish news reports said the hand of the suspected female attacker was found about 300 yards from the blast site.

In northern Iraq, warplanes struck at least 18 PKK positions, including bases in the Qandil mountains, Turkey’s state-run Anadolu Agency reported. Targets included ammunition depots, bunkers and shelters, the news agency said.

The blast on Sunday occurred less than a mile from the site of a Feb. 17 car bombing, which targeted a bus full of Turkish soldiers, killing 28 of them. An offshoot of the PKK asserted responsibility for that explosion.

Smaller-scale attacks against Turkish military targets have been commonplace in the largely Kurdish southeast since a cease-fire broke down last summer. The two recent attacks, however, suggest that the militants are seeking to escalate the fight by taking it into the heart of the country and hitting civilians as well.

The U.S. Embassy warned Friday in a message to American citizens that a terrorist attack might be imminent in Ankara, but it did not identify any group. A State Department statement condemned the latest bloodshed, saying that the United States remains committed to “combating the shared threat of terrorism” with NATO ally Turkey.

The Islamic State also has carried out attacks in Turkey in recent months. The worst killed more than 100 people in Ankara at a Kurdish peace rally in October. In Istanbul, 12 people, most of them German tourists, died after a suicide bomber in January struck the historic Sultanahmet district.

At the same time, Turkey also has bombed sites of a Kurdish group based in Syria, claiming the U.S.-backed fighters seek to make territorial gains as part of their push against the Islamic State.

The U.S. government, like Turkey, has designated the PKK a terrorist organization. But Washington has refused Turkey’s demands to add the Syrian Kurds to the list, saying it regards the group as a vital ally in the fight against the Islamic State.

Murphy reported from Washington.

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