Eighteen people were killed Sunday when a van packed with five tonnes of explosives blew up in Turkey's restive southeast, the prime minister said, in an attack blamed on Kurdish militants.

The bombing, which killed 10 soldiers and eight civilians, was one of the deadliest attacks on Turkish security forces since the attempted coup of July 15 when a rogue military faction tried to oust President Recep Tayyip Erdogan.

"The attack was perpetrated by a suicide bomber who detonated a van (packed) with five tonnes of explosives," Prime Minister Binali Yildirim told a news conference in Istanbul.

The attack targeted a security post in Hakkari province as security forces were searching vehicles in Semdinli district, the official news agency Anadolu said.

In a statement, the Hakkari governor said a vehicle refused to stop as it approached a checkpoint, resulting in soldiers responding with gunfire.

Militants then also started shooting, the governor's office said, quoted by Dogan news agency.

Another 27 people were injured in the blast caused by the outlawed Kurdistan Workers' Party (PKK), Erdogan said in a written statement. Anadolu reported at least 16 of those were civilians.

Soon after the attack, which Anadolu said left a seven-metre (22-foot) deep hole in the road, the military confirmed it had begun a large-scale air operation which the governor's office said was launched to "neutralise" PKK militants.

The governor said commando units on the ground were continuing to search for PKK fighters.

- 'Heinous terrorist attack' -
Deputy Prime Minister Numan Kurtulmus described Sunday's incident as an "atrocious" and "heinous terrorist attack", vowing on Twitter that Turkey would never surrender to militant groups.

The White House said it condemned "in the strongest terms the deadly terrorist attack today against a military checkpoint in southeastern Turkey that left many dead, including civilians, and scores more wounded."

"We remain steadfast in our support for our NATO ally, Turkey, and reaffirm our commitment to continue working together to defeat all forms of terrorism," Ned Price, the White House National Security Council spokesman said.

Over the past two months, the military says it has killed a total of 387 PKK militants in Hakkari province, CNN-Turk reported.

The PKK has waged a 32-year insurgency against the Turkish state, which has left nearly 40,000 dead since 1984. The group is proscribed as a terrorist organisation by Turkey, the United States and the European Union.

Since the collapse of a two-and-a-half-year ceasefire in July 2015, more than 600 security forces and 7,000 PKK militants have been killed, according to Anadolu.

Over the past 15 months, attacks on the Turkish security forces have continued on an almost daily basis as the government has pressed military operations against the PKK to rid urban areas of fighters.

Hakkari is a flashpoint in the renewed conflict. On Saturday the Turkish armed forces said it "neutralised" eight PKK militants in Cukurcu district after clashes with the group, Anadolu reported.

- Fight will go on -
Yildirim vowed that Turkey would continue with determination its "fight against the separatist terrorist organisation (PKK)... and all kinds of terrorist organisations" including jihadists from the Islamic State (IS) group.

Meanwhile, Erdogan promised: "The state with all its institutions, hand in hand with the people, is determined to stop the actions of the separatist terrorist organisation (PKK)."

The bombing comes a day after two suspects believed to have been preparing a car bomb attack blew themselves up on the outskirts of Ankara when police ordered them to surrender.

Turkish officials said they believed the pair were linked to the PKK.

The attack also took place a day before the year anniversary of the bloodiest attack in Turkey's modern history when 103 people were killed and more than 500 wounded in twin suicide bombings targeting a pro-Kurdish peace rally in the capital, Ankara.

That attack was blamed on IS.