With Islamic State near defeat in Syria, Damascus is setting its sights on territory held by Kurdish-led forces including eastern oil fields, risking a new confrontation that could draw the United States in more deeply and complicate Russian diplomacy.

President Bashar al-Assad and his Iranian allies appear to have been emboldened by events in Iraq, where Kurdish authorities have suffered a major blow since regional states mobilised against their independence referendum, analysts say.

Rivalry between the Kurdish-led Syrian Democratic Forces (SDF), supported by the United States, and the Syrian government backed by Iran and Russia is emerging as a fault line with their common enemy - Islamic State - close to collapse in Syria.

Syria's main Kurdish groups hope for a new phase of negotiations that will shore up their autonomy in northern Syria. Assad's government, however, is asserting its claim to areas captured by the SDF from the jihadist group, known in Arabic by its enemies as Daesh, in more forceful terms.

On Sunday, Damascus declared Islamic State's former capital at Raqqa would be considered "occupied" until the Syrian army took control - a challenge to Washington which helped the SDF capture the city in months of fighting.

And the eastern oil fields seized by the SDF in October, including Syria's largest, will be a target for the government as it tries to recover resources needed for reconstructing areas it controls, according to a Syrian official and a non-Syrian commander in the alliance fighting in support of Assad.

"The message is very clear to the SDF militants and their backers in the coalition, headed by America: the lands they took from Daesh are rightfully the Syrian state's," said the non-Syrian commander, who requested that his name and nationality be withheld.

"Regarding the resources of the Syrian people in the east - oil and so on - we will not allow anyone to continue to control the country's resources and to create cantons or to think about self government," added the commander, who is part of a military alliance that includes numerousIran-backed Shi'ite militias from across the region.

The Syrian official said the SDF could not keep control of oil resources. "We won't permit it," said the official, speaking to Reuters on condition of anonymity as he was giving a personal view.

The United States has not spelt out how military support for the SDF will evolve after Islamic State's defeat, a sensitive point due to the concerns of its NATO ally Turkey.

Ankara regards Syrian Kurdish power as a threat its national security as its forces are fighting Kurdish PKK rebels over the border in Turkey.

The U.S.-led coalition, which has established several military bases in northern Syria, has been helping the SDF shore up control of the recently captured al-Omar oil field in Deir al-Zor province.

"Many people will say that will help them with (political) negotiations, but only if the United States remains with them, otherwise they are going to get clobbered," said Joshua Landis, an expert on Syria and head of the Center for Middle East Studies at the University of Oklahoma.

"I think the Syrian government is going to push on some of these oil wells, in the same way as Iraq just pushed to get Kirkuk oil, and in the same way the Iraqi push is going to embolden the Syrian army," he said.

Kirkuk "lesson"

Iraqi Kurds took control of large areas outside their autonomous region during the fight against Islamic State. However, last month's independence referendum prompted Western opposition and fierce resistance from Baghdad, Ankara and Assad's Iranian allies, and the Kurdish authorities have since lost much territory to Baghdad, including oil producing areas around the city of Kirkuk.

The Syrian official said this should serve "as a lesson for the Kurds in Syria, so they think about the future".

Regional sources say the U.S. unwillingness to stop Iraqi government forces, backed by Shi'ite militias, from recapturing Kirkuk sent an encouraging message to Assad and his Iranian allies to retake the SDF-held oil areas in Syria.

With critical military support from Russia and the Iran-backed militias, Assad has recovered swathes of central and eastern Syria from Islamic State this year, having defeated many anti-Assad rebel factions in western Syria.

The Kurdish YPG militia, the dominant force in the SDF, controls the second largest chunk of Syrian territory - around a quarter of the country. Syrian Kurdish leaders say they are not seeking secession.

The YPG and Damascus have mostly avoided conflict during the Syrian civil war, setting aside historic enmity to fight shared foes. Kurdish-led regions of northern Syria have meanwhile focused on establishing an autonomous government which they aim to safeguard.

Moscow has called for a new "congress" of Syrian groups that may start work on a new constitution. RIA news agency cited a source close to peace talks as saying the first meeting would be held in the Russian Black Sea resort of Sochi on Nov. 18.

A Syrian Kurdish official told Reuters the administration in northern Syria had been invited to the congress. Kurdish officials said they discussed their political demands with the Russians as recently as last month.

A senior Kurdish politician said government statements directed at the Kurdish-led regions of northern Syria were contradictory, noting that the Syrian foreign minister had said in September that Kurdish autonomy demands were negotiable.

"One day they say we are willing to negotiate and then someone else denies this or puts out an opposing statement," Fawza Youssef said in a telephone interview with Reuters. "One of them declares war and the other wants to come negotiate. What is the regime's strategy? Dialogue or war?"

After the final defeat of Islamic State in Deir al-Zor, "the situation will drive all the political sides and the combatants to start the stage of negotiations", Youssef said.

The SDF has also pushed into Arab majority areas, including Raqqa and parts of Deir al-Zor, where it is working to establish its model of multi-ethnic local governance.

Analysts believe the Syrian Kurdish groups could use the SDF-held Arab areas as bargaining chips in negotiations with Damascus.

"There is no other option than to negotiate," Youssef said. "Either a new stage of tensions and attrition will start - which we are 100 percent against - or a stage of dialogue and negotiations will start."