The Islamic State of Iraq and the Levant reportedly killed five Turkish soldiers and injured several more in an attack in northern Syria, while Turkey announced that it is no longer "realistic" to exclude Syrian President Bashar al-Assad from a solution to the country's five-year civil war.

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) killed five Turkish soldiers and injured at least nine in a bombing in al-Bab, where Turkish-backed rebels have suffered casualties in a weeks-long bid to retake the town, according to the Dogan news agency.

Also on Friday, Turkish Deputy Prime Minister Mehmet Simsek said it was no longer "realistic" to insist on a solution to the Syria conflict without Assad.

Ankara acknowledged last year that Assad is an actor in Syria but it is the first time a senior Turkish official has openly said it would be unrealistic to insist on the embattled leader's departure for any solution.

"We have to be pragmatic, realistic. The facts on the ground have changed dramatically," Simsek told a panel on Syria at the World Economic Forum in Davos, Switzerland.

"Turkey can no longer insist on a settlement without Assad. It is not realistic."

Turkey, a vocal critic of Assad, has backed Syrian opposition rebels fighting to oust him since the complex conflict began with anti-government demonstrations in March 2011.

'Blame squarely on Assad'

"As far as our position on Assad is concerned, we think that the suffering of Syrian people and tragedies clearly ... the blame is squarely on Assad," Simsek said.

Turkey and Russia, which backs the Syrian government with military support, brokered a ceasefire deal between Assad's forces and rebel groups in late December, but violence has recently escalated across the country, particularly around the capital.

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Turkey, together with Russia and Iran, are convening talks next week in the Kazakh capital of Astana to shore up a ceasefire in Syria.

At Davos, Simsek said there had to be "a beginning in Astana" to make sure the conflict stops.

"For now at least the fighting has stopped, it is very, very critical because that is the beginning of anything else. The process is to make sure that we translate the current lull into a more lasting ceasefire initially, and then of course talk about more mundane stuff, settling the conflict."

Five years of fighting has left hundreds of thousands of Syrians dead, while more than half of the country's prewar population has been displaced internally or fled the country.

Elsewhere on Friday, the United Nations condemned ISIL for the demolition of two monuments on sites in Syria's ancient city of Palmyra, calling the destruction a "war crime" and "cultural cleansing".

In a statement, Irina Bokova, director general of the Paris-based UN cultural agency UNESCO, described the wrecking as "a new war crime and an immense loss for the Syrian people and for humanity."

"This new blow against cultural heritage... shows that cultural cleansing led by violent extremists is seeking to destroy both human lives and historical monuments in order to deprive the Syrian people of its past and its future," Bokova said.

ISIL recaptured Palmyra, a UNESCO World Heritage site, from government forces on December 11 and the new devastation reportedly occurred earlier this month.