Beirut (AFP) - The Islamic State jihadist group has released a video purportedly showing two captured Turkish soldiers being burned alive, after Ankara vowed to fight "terror" in Syria in response to 16 of its troops being killed in battle.

The 19-minute video, showing two uniformed men being hauled from a cage before being bound and torched, was posted on jihadist websites and was supposedly shot in the IS-declared "Aleppo Province" in northern Syria.

Speaking in Turkish, the killer of the two men criticises Turkish President Recep Tayyip Erdogan and calls for "destruction to be sowed" in Turkey.

The shocking images recall the killing of Maaz al-Kassasbeh, a Jordanian fighter pilot, who was captured by the jihadists when his plane went down in Syria in December 2014, and was later burned alive in a cage.

The IS-linked news agency Amaq said last month that the jihadists had kidnapped two Turkish soldiers, and the Turkish army separately said it had lost contact with two of its men.

The video's release comes a day after 16 Turkish soldiers were killed by IS fighters in Ankara's biggest loss so far in its unprecedented incursion into Syria.

They were killed in a succession of attacks around the Syrian town of Al-Bab on Wednesday that included three suicide car bombings.

The heavy toll showed the intensifying battle for the town, which Turkish forces have been seeking to capture for weeks in the biggest test of their four-month incursion into Syria.

Turkish troops entered Syria on August 24 in support of pro-Ankara Syrian rebels, with the aim of ousting IS jihadists as well as Kurdish militia from the border area.

At least 38 Turkish soldiers have been killed in the operation, which the Turkish government has dubbed Euphrates Shield.

Speaking earlier Thursday, Erdogan vowed no let-up in the ongoing campaign.

"Yes, maybe we will have to lay martyrs to rest," he said in a speech in Ankara.

"But we are determined to preserve their memory and protect what they left us and continue this struggle."

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Turkey, he said, "is engaged in its most serious struggle since the war of independence" that led to the creation of the modern state in 1923.

Turkish television showed distraught relatives of the dead dealing with the news and putting national flags outside their homes.

- 'Difficult fight' -

The earlier stages of Turkey's campaign proceeded with lightning speed and the border town of Jarabulus was taken on the first day of the offensive.

But the army has suffered increasing casualties in the fight for Al-Bab -- 25 kilometres (15 miles) from the border.

Defence Minister Fikri Isik told parliament on Thursday that 1,005 IS jihadists and 299 fighters affiliated to the Kurdish Peoples' Protection Units (YPG) had been killed in the operation so far.

Ankara considers the YPG a terror group, even though it works together with the United States as an ally in the fight against IS.

The army said the latest clashes erupted around a weapons depot that had been used by IS for the last two years.

Al-Bab lies 35 kilometres northeast of Aleppo, which is now under control of government forces in the biggest defeat for rebels in the civil war.

Turkey has been a key backer of the rebels and insists the ouster of President Bashar al-Assad is the only way to bring peace to Syria.

But Ankara has stayed out of the most recent battle for Aleppo and worked with Assad's key ally Russia to broker evacuations from the city.

Turkish air strikes on Al-Bab meanwhile killed at least 47 civilians including 14 children and nine women, the Syrian Observatory for Human Rights said.

There was no immediate response from the government to the claim.

Turkey has also been hit at home by the bloodiest attacks in its modern history, which it blames on jihadists and Kurdish militants.

The government is also carrying out a wide-ranging crackdown following an attempted coup in July, which it says was orchestrated by the group of an exiled cleric, Fethullah Gulen.