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

A car bomb in the heart of the Turkish capital, Ankara, killed at least 37 people Sunday, heightening anxieties that violence from the war against Kurdish militants in the southeast is spilling into Turkish cities.

The early-evening explosion struck near a busy square along Ataturk Boulevard, an area of malls and restaurants that is typically packed with shoppers and commuters.

It was the second such bombing in Ankara in less than four weeks and occurred less than a mile from the earlier one. The previous attack, on Feb. 17, targeted a bus full of Turkish soldiers, killing 28 of them, and was claimed by an offshoot of the Kurdistan Workers’ Party, or PKK.

There was no immediate assertion of responsibility for Sunday’s blast, but the pro-government Daily Sabah quoted a Turkish government official as saying the PKK or one of its affiliates is the main suspect.

The exact target wasn’t clear, and some Turkish media reports suggested it may have been a group of riot police, a common sight on the streets of Turkish cities, that had gathered nearby. CCTV footage showed a car parked on the busy boulevard exploding in a ball of flame as traffic whizzed past.

But initial reports suggested at least some of the casualties were civilians waiting at nearby bus stops, which would mark another worrying twist in the escalating violence between the Turkish government and the PKK.

The Health Ministry said 37 people were killed and at least 128 were injured in Sunday’s bombing.

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

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.

The Islamic State has also carried out attacks in Turkey in recent months, but those were suicide bombings, not car bombs. The worst killed more than 100 people in Ankara at a Kurdish peace rally last October. In Istanbul, 11 people, most of them German tourists, died in January when a suicide bomber struck the historic Sultanahmet district.

The violence has unnerved Turkey, which finds itself entangled in fights on two fronts, against the Kurds in southeastern Turkey and against the Islamic State in Syria. The two wars are becoming ever more closely intertwined, with Turkey firing artillery into Syria to halt advances there by the Syrian Kurdish People’s Protection Units, or YPG, which is allied with the PKK, against the Islamic State and Syrian rebels.

The fighting between Turkey and the YPG has caused friction with the United States, too, which is allied with both of them in the war against the Islamic State.

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

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