By Angus McDowall and Tom Perry
BEIRUT (Reuters) - Syrian rebels backed by Turkey were poised to begin an assault to try to drive Islamic State from the Syrian city of al-Bab, two of their commanders said on Monday, a battle that could also prompt new fighting with Kurdish groups that are competing for the area.
Further escalation in the complex, multi-sided conflict in northern Syria has the potential to undermine a campaign supported by an international coalition led by the United States to oust Islamic State from its Syrian capital of Raqqa.
Al-Bab is fast becoming a major faultline in the war in northern Syria, bringing Free Syrian Army rebels backed by Turkish armor closer than ever to frontlines held by the Syrian government and its Iranian and Russian allies in nearby Aleppo.
"There is nothing between us and al-Bab," said one of the rebels, a commander in one of the groups fighting under the Free Syrian Army (FSA) banner taking part in the Turkey-backed Euphrates Shield operation in north Syria that began in August.
"If not in hours then in a very few days we will be inside al-Bab," the commander told Reuters, declining to be identified.
The Britain-based war monitor, the Syrian Observatory for Human Rights on Monday cited residents saying areas surrounding al-Bab were being struck by Turkish and FSA artillery. Another FSA commander who identified himself by the nom de guerre, Abu Assad Dabiq, said rebels were less than 3 km (2 miles) away.
The main struggle in Syria's civil war pits President Bashar al-Assad, backed by Russia, Iran and Shi'ite militias against Sunni rebels backed by Turkey, the United States and Gulf monarchies. They are all fighting against Islamic State.
Further complicating the warfare in northern Syria, Turkey and the FSA rebel groups it backs are also set against a Kurdish-dominated alliance of militias that has fought Islamic State since 2015 and recently began a campaign to take Raqqa.
GOVERNMENT ALLIES' WARNING
Al-Bab is located 30 km (19 miles) south of Syria's border with Turkey and the same distance from Aleppo, meaning its capture could help rebels to advance against pro-government forces besieging their comrades inside the city.
The city of al-Bab had a population of 63,000 before the war according to a 2004 census by Syria's Central Bureau of Statistics and is a hub for major roads in the region north of Aleppo.
"God willing with the capture of al-Bab, we will be on the outskirts of the (Aleppo) industrial zone and the outskirts of the Kweiras air port, and the outskirts of the infantry college, meaning in direct contact with the regime," the rebel commander, who declined to be identified, said.
However, allies of the Syrian government late last month warned Turkey against an advance towards their positions to the north and east of Aleppo, saying any such move would be met "decisively and with force".
The Syrian government is backed by the Iranian Revolutionary Guards, Lebanon's Hezbollah and other Shi'ite militia.
Al-Bab also sits between two Kurdish-ruled enclaves and its capture would thwart Kurdish ambitions to join them, something some Syrian Kurds regard as necessary to advance their goal of protecting Kurdish autonomy in northern Syria.
While Turkey's launch of the Euphrates Shield campaign in August was partly aimed at pushing Islamic State from border areas after the jihadist group shelled Turkish towns, it was also intended to prevent the Kurdish enclaves from joining.
Ankara regards the main Kurdish militia group YPG as being an extension of the Kurdistan Workers Party (PKK), which has fought a three-decade insurgency inside Turkey and is listed by the United States and European Union as a terrorist group.
The YPG and allied groups have also tried to advance towards al-Bab in recent months from their two enclaves to the west and east but remain about 20 km away on each side.
The YPG is the strongest element of the Syrian Democratic Forces (SDF), a U.S.-backed alliance of armed groups fighting against Islamic State which last week began an offensive against territory held by the jihadist group north of Raqqa.
Turkey and the YPG have both insisted that the other should have no part in the Raqqa campaign.
The push against Raqqa has already been complicated by concerns that the central role of Kurds in the battle for the mostly Arab city would play into Islamic State propaganda.
Last week, the Liwa Thuwwar al-Raqqa, one of the few Arab groups involved in the fighting in that area said it would not take part in the campaign because there were too few Arabs involved.
"We are not satisfied that the campaign should start this way," Mahmoud al-Hadi, head of the group's political office, told Reuters.
Jihan Sheikh Ahmed, the spokeswoman for the SDF's Raqqa offensive, in comments to reporters on a social networking site on Saturday said she believed there were "sufficient" Arabs taking part in the campaign.
The United States has said Arabs must play the central role in any push to take Raqqa.
Turkey has also said that after the Euphrates Shield operation captures al-Bab, it will target Manbij, a city 40 km to the east that has been held by forces allied to the SDF for months.
That could prompt a much wider escalation between Turkey and Kurdish groups that could in turn foil efforts by the United States to orchestrate an SDF assault on Islamic State in Raqqa.
"As for what is happening in al-Bab, it has an impact one way or another on our campaign," said Ahmed of the Raqqa offensive.
(Reporting By Tom Perry, Ellen Francis and Angus McDowall in Beirut and Suleiman al-Khalidi in Amman, editing by Peter Millership)