The death toll from a car bombing at a crowded transport hub in Turkey's capital Ankara has risen to at least 34, according to the country's health minister.

At least 125 more were wounded in the second such attack in the administrative heart of the city in less than a month.

The blast, which could be heard several kilometres away, sent burning debris showering down over an area a few hundred metres from the Justice and Interior Ministries, a top courthouse, and the former office of the prime minister.

One senior security official said initial findings suggested the attack had been carried out by the Kurdistan Workers Party (PKK) or an affiliated militant group, but there was no immediate claim of responsibility.

The previous car bombing came just a few blocks away on 17 February, which killed 29 people, most of them soldiers, and was claimed by a group close to the PKK.

The government said Syrian Kurdish militants were also involved in that attack, near the military headquarters, parliament and other key government institutions.

Prime Minister Ahmet Davutoglu held an emergency meeting with the interior minister, the head of the intelligence agency and police and security chiefs, officials said. President Tayyip Erdogan spoke by phone with the interior minister.

A second senior official said gunfire was heard after the blast.

State broadcaster TRT said the car had exploded at a major transport hub, hitting a bus carrying around 20 people near the central Guven Park and Kizilay Square. It said the area was crowded when the explosion happened.

NATO member Turkey faces multiple security threats. As part of a US-led coalition, it is fighting the so-called Islamic State in neighbouring Syria and Iraq.

It is also battling PKK militants in its south east, where a two-year ceasefire collapsed last July, triggering the worst violence since the 1990s.

Turkey sees the unrest in its largely Kurdish south east as deeply linked to events in northern Syria, where the Kurdish YPG militia has been seizing territory as it fights both IS and rebels battling President Bashar al-Assad.

Ankara fears those gains will stoke separatist ambitions among its own Kurds and has long argued that the YPG and PKK have close ideological and operational ties.

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

The Kurdistan Freedom Hawks (TAK), which experts say is a direct affiliate of the PKK, claimed responsibility for the 17 February attack. TAK itself said it split from the PKK.