BEIRUT (AP) — Syrian troops battling the Islamic State militant group crossed to the eastern bank of the Euphrates River in Deir el-Zour on Monday, threatening a potential faceoff with the U.S.-backed forces operating there.

But the U.S.-led coalition spokesman played down concerns, saying both advancing forces are maintaining lines of communication to avoid conflict.

Col. Ryan Dillon, spokesman for the U.S.-led coalition, told The Associated Press that the top commander of the U.S.-backed Syrian forces has maintained "open lines" of communication and discussions with his counterpart in the Syrian government to avoid friction.

This is the first sign of direct contact between the U.S.-backed Syrian Democratic Forces and the forces of Syrian President Bashar Assad, as the race for control of the oil-rich province picks up speed.

Dillon said "de-confliction" talks with Russia have increased in frequency as well as at different operational levels, to avoid ground friction as the forces "converge and have a common goal together" to fight IS.

Dillon's comments came after Syrian troops crossed to the eastern side of the Euphrates river that runs along Deir el-Zour city, where the SDF have been waging their separate offensive against IS. The crossing came two days after the SDF said its fighters were hit by Russian airstrikes on the eastern bank of the river. Russia denied it was behind the airstrikes.

At the Pentagon, Defense Secretary Jim Mattis said U.S.-Russia "de-confliction" arrangements for preventing unintended conflict between U.S. and Russia in Syria had been working well prior to the Russian airstrikes on SDF fighters.

"They've been in place, they've been working, and this is a departure," he said of the attack.

Asked what is being done about it, he said: "We are sorting it out. We have active communications, both State and Defense (departments), with our counterparts over the weekend and going into tomorrow."

Both sides are seeking to expand their control of the oil-rich province bordering Iraq. Many oil fields, including al-Omar, Syria's largest, are scattered on the eastern bank.

The U.S.-backed offensive is focused on the Iraq border area, which is still controlled by IS. Washington fears that further advances by pro-government forces could help Iran expand its influence across the region via a land bridge stretching through Iraq, Syria and Lebanon, all the way to Israel.

Iran is a key ally of Assad, and has provided financial and logistical support to his forces throughout the six-year civil war.

Dillon said the government forces have advanced "quickly" to seize Deir el-Zour city, but it remains to be seen if they "have what it takes" to hold the newly liberated territories. He said the U.S.-backed fighters have no intention advancing on the city.

The crossing tightens the government's grip on the city by closing in on the militants from all sides, except for a narrow waterway flowing south. It comes nearly two weeks after the Russia-backed Syrian troops breached a three-year siege on their troops in the city.

IS militants still have pockets inside the city, one of their last major strongholds in Syria, on the western and eastern banks of the river. There are nearly 100,000 residents in Deir el-Zour.

The pro-government forces meanwhile announced that they had secured the airport in Deir el-Zour, allowing two military transport aircraft to land. Al-Manar TV, the media arm of Lebanon's Hezbollah, quoted an unnamed general who said the airport is "90 percent secured." Hezbollah is fighting alongside Assad's forces.

Russia, which has provided crucial air support to Assad's forces, said Syrian elite troops used pontoon bridges to cross the river.

"The Syrian army storm units have pushed the IS militants out of several villages on the eastern bank of the Euphrates River and are continuing their offensive toward the east, extending their gains."

Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights, said the crossing was preceded by intense aerial bombing on the eastern bank. He said the government forces are now in Marrat, battling IS militants in Hatlah to the north.

"Even if the pro-government (forces) keep up their advance in the city, it will mean nothing if they don't control the eastern bank," Abdurrahman said.

SDF spokesman in Deir el-Zour, Bassem Aziz, said only miles of IS-controlled territory separated them from government forces on the eastern bank.

He said his forces will avoid any clashes as SDF commanders convened to review their plans but that the eastern banks of the Euphrates river remain the focus of their advances.

The town of al-Mayadeen, south along the river, has become the main hub for IS leadership as the group's strongholds of Raqqa and Deir el-Zour have come under attack. Further south, Boukamal sits on the border between Iraq and Syria and would be crucial in any future plans to control the route between the two countries.

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Associated Press writers Bassem Mroue in Beirut, Vladimir Isachenkov in Moscow and Robert Burns in Washington contributed to this report.