Kurdish fighters engaged in fierce clashes with fighters on the Turkish border near Kobane on Monday, as Ankara denied allowing Washington to use its bases against the Islamic State group.

US officials had earlier said Turkey would also host training for "moderate" Syrian rebels, in the hopes of finally creating a force capable of tackling IS on the ground.

"There is no new agreement with the United States about Incirlik," a Turkish government official told AFP of an air base in southern Turkey that the US wants to use to launch air strikes.

"Negotiations are continuing" based on Turkish conditions previously laid out, the official added.

Near Kobane, fighting spread early Monday to the border area just north of the flashpoint town, threatening to cut it off from the outside world.

IS fighters were also putting strong pressure on pro-government forces in Iraq, with concern over Anbar province and the key oil refinery of Baiji.

With the fighters advancing on its doorstep, NATO member Turkey has come under intense pressure to take action as part of a US-led coalition that has been carrying out air strikes in Syria and Iraq.

US officials said Turkey had agreed to let Washington use its bases including Incirlik for the air campaign.

Pentagon chief Chuck Hagel said the agreement included "hosting and conducting training for Syrian opposition members" in Turkey, noting that Ankara would welcome a US Command team next week to "develop a training regimen".

US military planners have repeatedly warned that the air campaign alone will not be enough to defeat IS, which in June declared an Islamic "caliphate" in the large parts of Syria and Iraq under its control.

Turkish Prime Minister Ahmet Davutoglu on Sunday called for military backing for Syria's "moderate opposition" to create a "third force" in the war-torn country to take on the Damascus regime as well as IS militants.

Iraq forces leave Anbar base

Near Kobane, clashes with automatic gunfire and mortar fire were taking place less than a kilometre (half a mile) from the barbed wire fence that marks the border between Syria and Turkey, an AFP reporter on the Turkish side said.

Turkey had moved reinforcements to the border including more tanks and self-propelled artillery, the reporter said.

The US-led international coalition launched at least two new air strikes against IS positions in the area, with one striking the heart of the town and sending a huge plume of smoke skywards.

Kobane has become a highly visible symbol of resistance to IS and its fall would give the fighters control of a long stretch of the Turkey-Syria border.

But concern has also been growing over Iraq, where IS fighters have been threatening to seize more territory.

Iraqi forces are reported to be under intensifying pressure in Anbar province between Baghdad and the Syrian border, where a roadside bomb killed the police chief on Sunday.

On Monday, security sources said Iraqi government troops stationed on the edge of the city of Heet in Anbar had withdrawn to another base, leaving the city under full fighters control.

Pro-government forces have also been in trouble around Baiji oil refinery south of IS-held Mosul, where US military aircraft on Sunday for the first time dropped supplies including food, water and ammunition to Iraqi troops.

Washington has insisted it will not send ground troops back to Iraq and Secretary of State John Kerry said Sunday the Iraqis themselves will have to succeed on the ground.

"Ultimately it is Iraqis who will have to take back Iraq. It is Iraqis in Anbar who will have to fight for Anbar," he said in Cairo.

Fighters divide up 'slaves'

IS has committed widespread atrocities in areas under its control, including attacks on civilians, mass executions, beheadings and enslaving women.

In the latest issue of its propaganda magazine Dabiq released on Sunday, IS boasted of having revived slavery, giving Yazidi women and children captured in northern Iraq to its fighters as spoils of war.

IS believes the Yazidis hold deviant religious beliefs and claims that Islamic sharia law allows for their enslavement.

"After capture, the Yazidi women and children were then divided according to the sharia amongst the fighters of the Islamic State," the article said.

The group has also murdered four Western hostages in on-camera beheadings, and on Sunday hundreds of people gathered in northwest England for a memorial service for British aid volunteer Alan Henning.

The 47-year-old taxi driver had travelled to Syria to help Muslim colleagues deliver aid in a convoy, but was kidnapped and his murder claimed by IS in a graphic video released on October 3.

More than 180,000 people have been killed in Syria since an uprising against President Bashar al-Assad's regime began in 2011, evolving into a several-sided civil war that has drawn thousands of fighters from overseas.