ISTANBUL: Turkey Monday said it wanted international air support for its army's assault on the militant-held Syrian town of al-Bab where the military is facing tough resistance from ISIS extremists.

Turkish forces have for weeks joined pro-Ankara Syrian rebels in fierce fighting for al-Bab, taking increasing casualties as they approach closer to the center.

Turkey is part of the U.S.-led coalition against ISIS militants in Syria and lets Western war planes use its Incirlik air base as a hub for air raids.

"As for our operations in al-Bab, the international coalition should assume its responsibilities, especially where air support is concerned," presidential spokesman Ibrahim Kalin said in televised comments in Ankara.

"The weather conditions can sometimes entail delays," Kalin acknowledged. "But the absence of air support when there is no valid reason is unacceptable," he added.

The U.S. and its allies have been conducting their own airstrikes against ISIS targets in Syria but there have been no reports of them specifically aiding the Turkish operation.

Turkey at the weekend deployed more tanks and artillery to the border and also has sent 500 elite commandos to al-Bab in readiness for a final fight for the town, reports said.

A Britain-based, opposition-aligned group has accused Turkey of killing 88 civilians in airstrikes on al-Bab. However the army has unequivocally denied such claims.

Thirty-six Turkish soldiers have died so far in the operation inside Syria – dubbed Euphrates Shield – since it was launched on Aug. 24.

ISIS fighters last week circulated a video purportedly showing two Turkish soldiers captured by the extremists in Syria being burned alive.

The authenticity of the video cannot be confirmed and Turkey's leadership has made no mention of the images.

Turkey's Defense Minister Fikri Isik has said three Turkish soldiers are being held by ISIS, without giving further detail. The issue was evoked at Kalin's televised news conference but the spokesman did not give any comment.

Users in Turkey had reported severe problems with social media after the video emerged. However by Monday access to Twitter was fully restored after three days of disruption.