BEIRUT: The Islamic State group killed three of its captives in Syria's ancient city of Palmyra by tying them to Roman-era columns at the site, then blowing the structures up with explosives, activists said Tuesday.

The Palmyra explosions were the latest method of killing by IS militants, known for beheadings, immolation and drowning of prisoners.

A Palmyra activist who goes by the name Nasser al-Thaer said that the killings of the three captives took place on Monday afternoon at the Palmyra archaeological site, located a few kilometers (miles) away from the city. Al-Thaer and the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said the three were civilians but that their identities remain unknown.

Earlier this week, IS posted images on social media purported to show its members driving a tank over a captured government soldier, allegedly to revenge what it said was his driving over IS militants. The group is known to have tanks, mostly captured in battle from Syrian troops or in the territory it holds in neighboring Iraq.

IS has also destroyed many of the ancient Palmyra's Roman-era relics, including the magnificent Temple of Bel and the iconic Arch of Triumph. IS captured Palmyra from government forces in May, and considers such relics as promoting idolatry. But scientists say the group resorts to promoting and documenting such attacks also for their shock value, to spread awe and fear. The group has relied on looting and selling such antiquities on the black market for revenue.

Also Tuesday, Turkey confirmed that its military this week attacked the main Kurdish force across the border in northern Syria, a key ally of the United States in its efforts to defeat the Islamic State.

The US-supported Kurdish militia, known as the YPG, said the Turkish military shot at its forces deployed in the town of Tal Abyad twice on Sunday, using mostly machine guns. There were no casualties in the shooting and the Kurdish forces didn't return fire.

Turkish Prime Minister Ahmet Davutoglu confirmed that the military targeted the Kurdish forces in an interview with Turkey's ATV television late Monday.

But his account was different from the Kurdish force. He said the Turkish military attacked the Kurdish force west of the Euphrates river, while the YPG said the attack was in Tal Abyad, which is east of the river.

"We said the PYD will not go west of the Euphrates and that we would hit it the moment it did. We hit it twice," Davutoglu said. The YPG is the fighting force of the PYD, or the Kurdish Democratic Party.

"Turkey cannot abandon its border, its fate to any country," he added, without elaborating.

Turkey is wary of the PYD, which is affiliated with Turkey's Kurdish PKK rebels, who have waged a bloody insurgency in southeastern Turkey. The Kurdish capture of the majority-Arab town of Tal Abyad, and its subsequent inclusion under the semi-autonomous enclave has further irked the country.

Turkey, which is part of the US-led coalition against IS, however differs with Washington on whether the Kurdish force is a terrorist organisation. Both countries have labeled the PKK as a terror group but only Turkey considers the Syrian Kurdish forces as terrorists.

Kurdish fighters expelled Islamic State militants from Tal Abyad in June, dealing a major blow to the extremist group's abilities to access supply routes across the Turkish borders.

Last week, Tal Abyad was declared an independent administration allied with the semi-autonomous Kurdish enclave in northern Syria.