The Islamic State in Iraq and Levant (ISIL) has launched a major attack on the Iraqi city of Ramadi, capital of the troubled western province of Anbar, security officials have said, resulting in the killing of at least 20 soldiers.

The assault came as Joe Biden, the US vice-president, arrived in Istanbul on Friday with a view to push Turkey to step up its role in the international coalition's fight against the ISIL.

Al Jazeera's Imran Khan, reporting from Baghdad, said pro-government forces had called in reinforcements to push back the offensive on Ramadi that was coming from four sides.

"Ramadi is a crucial city for ISIL as it attempts to consolidate its grip over all of Anbar province," Khan said.

Sources told Al Jazeera tens of Iraqi soldiers had been abducted near Ramadi while at least 20 Iraqi soldiers and eight ISIL fighters had been killed in the fighting.

"Clashes are ongoing around the city. A series of mortar attacks have targeted areas inside the city, including provincial council buildings and a police post," a security official told the AFP news agency said.

Adhal al-Fahdawi, a member of the Anbar provincial council, said on Friday that ISIL had managed to capture part of an eastern district called Mudhiq but pro-government forces had stopped their advance and were encircling the fighters there.

"The security forces need support because we have not received any back-up from the army's air force or the coalition," Fahdawi said, referring to the US-led air campaign launched in August.

Parts of the restive province, which borders Saudi Arabia, Jordan, Syria and the Baghdad governorate, have been out of government control since January.

ISIL, which also controls large parts of Syria, spearheaded a major offensive in Iraq in June, seizing territory, including much of Anbar.

A fresh spate of attacks in recent weeks has seen the armed group extend their grip over the province, where only a handful of pockets remain under the control of Iraqi security forces backed by Shia armed groups and Sunni tribal fighters.

US-Turkey talks

Biden met the Turkish Prime Minister Ahmet Davutogulu on Friday and will hold talks with the President Recep Tayyip Erdogan on Saturday.

Biden's visit follows weeks of public bickering between the two NATO allies. The Turkish president insists if the US wants his help, it must focus less on fighting the ISIL and more on toppling Syrian President Bashar Assad. Erdogan wants the US-led coalition to set up a security zone in northern Syria to give moderate fighters a place to recoup and launch attacks.

The obvious compromise would be if Washington shifted its policy on Syria to do more to force out Assad, and Turkey agreed to do more against ISIL, said James Jeffrey, former US ambassador to Turkey and Iraq who is now at the Washington Institute for Near East Policy.