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A car bombing in Turkey's capital has killed at least 34 people and wounded around 125 others on Sunday, officials said, the second such attack in the area in less than a month.

The blast, which could be heard several kilometers away, occurred on the city's main boulevard, Ataturk Bulvari, close to Ankara's main square, Kizilay. It happened adjacent to bus stops near a park.

The private NTV news channel said a car, believed to be laden with explosives, detonated close to a bus. Several vehicles then caught fire, it said. The area is close to government offices, including the justice and interior ministries, as well as a courthouse and the former office of the prime minister.

Dogan Asik, 28, said he was on a bus when the explosion occurred.

"We were thrown further back into the bus from the force of the explosion," said Asik, who sustained injuries on his face and arm.

Health Minister Mehmet Muezzinoglu said 30 of those killed had died at the scene, while the four others died in hospital. At least one or two of the dead were attackers, he said, and 19 of the 125 wounded were in critical condition.

Speaking hours after the bombing, Turkish President Tayyip Erdogan said that such attacks would only bolster Turkey's resolve in fighting terrorism and that it would never give up the right to defend itself.

Erdogan said terrorist organisations were targeting civilians because they were losing their struggle against the security forces. Turkey had become a target because of regional instability in recent years, he added.

Police sealed off the area and pushed onlookers and journalists back, warning there could be a second bomb.

Two senior security officials told Reuters the first findings suggested that the Kurdistan Workers Party (PKK), which has waged a three-decade insurgency for Kurdish autonomy, or an affiliated group, was responsible.

There was no immediate claim of responsibility, but Interior Minister Efkan Ala said the name of the group behind the attack would likely be announced on Monday after initial investigations were completed.

"Tonight, civilian citizens waiting at a bus stop were targeted in a terrorist attack with a bomb-laden car," Ala told reporters after a meeting with Prime Minister Ahmet Davutoglu, the head of the intelligence agency and security chiefs.

"Significant findings have been made, but the organisation behind this will be announced once the investigation has been finalised," he said.

One of the security officials said the car used in the attack was a BMW driven from Viransehir, a town in the largely Kurdish southeast, and that the PKK and the affiliated Kurdistan Freedom Hawks (TAK) appeared to be responsible.

TAK claimed responsibility for the previous car bombing, just a few blocks away, on Feb. 17. That attack targeted a military bus as it waited at traffic lights, and killed 29 people, most of them soldiers, near the military headquarters, parliament and other key government institutions.

A police source said there appeared to have been two attackers, one a man and the other a woman, whose severed hand was found 300 meters from the blast site.

The explosives were the same kind as those used on Feb. 17 and the bomb had been reinforced with pellets and nails to cause maximum damage, the source told Reuters.

Defne Karadeniz | Getty Images

Emergency workers at the scene of the bombing in Ankara's central Kizilay district.

The bombing is the third in the city in five months and comes as Turkey is faced with an array of issues, including renewed fighting with the Kurdish rebels, threats from the Islamic State group and a Syrian refugee crisis.

It occurred just three weeks after a suicide car bombing in the capital targeted buses carrying military personnel, killing 29 people. A Kurdish militant group which is an offshoot of the outlawed Kurdish rebel group, the Kurdistan Workers' Party, or PKK, claimed responsibility for the Feb. 17 attack. The government, however, said that attack was carried out by a Syrian Kurdish militia group in concert with the PKK, which has waged a 30-year insurgency.

Sunday's attack also came two days after the U.S. Embassy issued a security warning about a potential plot to attack Turkish government buildings and housing in one Ankara neighborhood and asked its citizens to avoid those areas.

As with the previous bombings, Turkish authorities quickly imposed a ban Sunday preventing media organizations from broadcasting or publishing graphic images of the blast or from the scene.

Hundreds of people have been killed in Turkey in renewed fighting following the collapse of the peace process between the government and the PKK in July. Authorities on Sunday had declared curfews in two towns in the mainly Kurdish southeast region in anticipation of large-scale military operations against PKK-linked militants.

Turkey also has been struck by several bombings in the last year that were blamed on IS as the government joined efforts led by the U.S. to fight the extremist group in Syria. The deadliest came in October when a peace rally outside Ankara's main train station killed 102 people.

The United States condemned the attack, saying in a White House National Security Council statement: "This horrific act is only the most recent of many terrorist attacks perpetrated against the Turkish people. The United States stands together with Turkey, a NATO ally and valued partner, as we confront the scourge of terrorism."