* Talks would show how radical fighters eclipsed moderates
* Diplomatic sources say U.S. envoy Ford due in Istanbul
* Opposition rivalry has hampered fight against Assad
Syrian rebel commanders from the Islamic Front which seized control of bases belonging to Western-backed rebels last week are due to hold talks with U.S. officials in Turkey in coming days, rebel and opposition sources said on Saturday.
The expected contacts between Washington and the radical fighters reflect the extent to which the Islamic Front alliance has eclipsed the more moderate Free Syrian Army brigades - which Western and Arab powers tried in vain to build into a force able to topple President Bashar al-Assad.
The talks could also decide the future direction of the Islamic Front, which is engaged in a standoff with yet more radical Sunni Muslim fighters from the al Qaeda-linked Islamic State in Iraq and the Levant (ISIL).
A rebel fighter with the Islamic Front said he expected the talks in Turkey to discuss whether the United States would help arm the front and assign to it responsibility for maintaining order in the rebel-held areas of northern Syria.
He declined to be identified because of the sensitivity of the talks, and gave no further details. Diplomatic sources in Turkey said that U.S. Syria envoy Robert Ford was expected in Istanbul soon but his schedule was not yet confirmed.
The Islamic Front, formed by the unification of six major Islamist groups last month, seized control a week ago of weapons stores nominally under the control of the Free Syrian Army's Supreme Military Command (SMC).
It has since said it was asked to take over the base by the SMC to protect it from attack by ISIL fighters. Whether or not the move was requested, it demonstrated how little power the Western-backed SMC wields in rebel-held Syria.
An SMC rebel commander also said he had been told the Islamic Front would hold talks with U.S. officials in Turkey in the coming days.
The infighting and rivalries among the rebels have undermined their fight against Assad in Syria's 2-1/2 year civil war, which has killed more than 125,000 people according to the Syrian Observatory for Human Rights.
The conflict has also reduced whole city districts across Syria to rubble, causing tens of billions of dollars of damage, driven 2 million refugees to seek safety abroad and made millions more homeless and vulnerable to a winter storm which has covered the region in snow and biting rain.
The Islamic Front rebel told Reuters that rivalry with the ISIL had already led to a spate of hostage-taking between the two sides, and that the Front's decision to talk to the Americans had further escalated tension.
Although he described the two Islamist forces as ideologically close, he said ISIL appeared set on confrontation, perhaps encouraged by some of their backers in Saudi Arabia.
"The front has to talk to ISIL via messengers because of the tense situation," he said. "ISIL sees things in black and white. They are very stubborn."
"So far the Islamic Front has been restraining itself, having some sort of dialogue with ISIL," the rebel said. But he said that unless the hostages were released soon "there will be more discussions and a different decision will be taken".
Contacts with the United States will not be undertaken lightly by the Islamic Front, which includes Salafi groups such as Ahrar al-Sham brigades which are mainly hostile to the West and have rejected U.S.-Russian backed U.N. peace talks for Syria, due to be held in Switzerland next month.
But their leaders have compared engaging with Washington to the Prophet Mohammad's temporary and tactical truces with enemy tribes as he built up his power.
The U.S. State Department, asked earlier this month whether it was in contact with Islamist rebels in Syria, said it wanted to work with a range of groups to try to persuade them to be part of the peace negotiations.
Rebels control a large region of northern and eastern Syria but have failed to unite in a single military force, allowing Assad's army to make some inroads around the northern city of Aleppo in recent weeks.
The army, backed by Lebanese Hezbollah guerrillas and Shi'ite Iraqi fighters, has also recaptured towns and suburbs around Damascus and along the main highway north from the capital towards the central city of Homs.
Last week's Islamic Front seizure of the SMC weapons bases led the United States and Britain to suspend non-lethal aid into northern Syria. But the opposition Syrian National Coalition said on Friday that more help, not less, was desperately needed.
"We know that we have a problem, we know that we don't have the organised military institutions that we want. We know of the challenges of the loose organisation of the Free Syrian Army," Coalition chief of staff Monzer Abkik said in London.
Appealing for international support to restructure the rebel forces, he said the alternative to an overhaul of their military operations was "complete chaos".
"There are many, many groups fighting the regime and fighting each other and fighting al Qaeda. It is a complete mess on the ground," he said.