The suicide bomber who attacked a wedding party in Gaziantep, near the border with Syria, was between the age of 12 and 14 years, Turkish President Recep Tayyip Erdogan said Sunday.

The bombing that hit the outdoor celebration late Saturday in the south-eastern Turkish city left 51 people dead, Erdogan said in raising the toll, according to broadcaster CNN Turk.

The pro-Kurdish Peoples' Democratic Party (HDP) said it was the wedding of one of its members.

At least 69 people were wounded and being treated in hospital, of who 17 were in critical condition, Erdogan told reporters in Istanbul.

The bride and groom - Besna and Nurettin Akdogan – survived the attack and were in hospital, but were not critically wounded, state-run Anadolu news agency reported.

"We couldn't see anything. Nothing but body parts," Sukru Akdogan, the groom's brother, was quoted by Anadolu as saying.

Gaziantep, a city of 1.5 million people, is about 40 kilometres from the Syrian border.

Erdogan blamed Islamic State for the attack. The extremist group, which controls large swathes of territory in Syria, has not claimed responsibility for the bombing.

"The initial findings of the governor and our police forces indicate the attack has been perpetrated by Daesh," Erdogan said, using an Arabic acronym for Islamic State.

"Daesh is trying to position and organize itself in Gaziantep. Security operations have been conducted and are still being conducted against the terrorist organization," he added, according to Anadolu.

The United States condemned the bombing in Turkey, with National Security Council spokesman Ned Price saying "the perpetrators of this barbaric act cynically and cowardly targeted a wedding, killing dozens and leaving scores wounded."

"We stand with the people of Turkey as they defend their democracy in the face of all forms of terrorism," Price said in a statement, calling Turkey "our valued NATO ally and partner."

Vice President Biden is scheduled to visit Ankara on August 24.

German Chancellor Angela Merkel said that once "again innocent men, women and children have become the victims of cowardly and malicious violence."

In a note of condolence to Turkish Prime Minister Binali Yildirim, she wrote: "Our thoughts are with the victims and their families ... I wish the wounded a speedy recovery."

"We condemn the barbaric attack on innocent civilians last night in Gaziantep," John Bass, the US ambassador to Turkey, said. "We stand by our ally Turkey and pledge to continue to work closely together to defeat the common threat of terrorism."

Pope Francis on Sunday offered prayers for the victims.

Speaking after the Angelus prayer in St Peter's Square, Francis said, "sad news has reached me about the bloody attack ... Let us pray for the victims, for the dead and the injured, and we ask for the gift of peace for all," according to Vatican Radio.

Images from the scene showed at least one structure on fire, bodies covered in blankets, and blood on the street and walls. Videos posted on social media also showed the chaos, with people using the light on their mobile phones to find victims.

Gulser Ates, who was wounded in the attack, told Anadolu: "I don't know what happened. The only thing I know is that my neighbour died on top of me.

"If she had not fallen on me, I would have died, too. Her body saved me ... There were innocent children there. No one had done anything wrong."

Erdogan said earlier that those who "provoke people by exploiting ethnic and sectarian sensitivities will not prevail," according to state broadcaster TRT.

Prosecutors told the Dogan news agency that the remains of an explosive vest had been found.

Islamic State has been ceding ground to US-backed Kurdish-led forces and recently lost Minbij, a key Syrian city near the border with Turkey, shrinking its areas of control.

Islamic State was blamed for an attack in May that killed two police officers in Gaziantep. The group has also been held responsible for other attacks in the country, including several in Istanbul.

In June, more than 40 people were killed in an attack at the city's airport blamed on the extremist group.

Turkey is also facing conflict in the south-east with Kurdish militants.

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