US-backed fighters in Syria said they were attacked by Russian jets for the first time in the complex civil war early Saturday. The allegation was later collaborated by the Pentagon, who added that western advisors were also in the area.

"Russian munitions impacted a location known to the Russians to contain Syrian Democratic Forces and coalition advisers," the Pentagon said in a statement.

"Several SDF fighters were wounded," the statement added. None of the advisers were injured.

But Russia's military spokesman denied Moscow targeted the Syrian Democratic Forces (SDF), an alliance of Arab and Kurdish fighters supported by Washington.

"This is not possible. Why would we bomb them?" military spokesman Igor Konashenkov said.

Both the SDF and the Russian-backed Syrian government troops are battling Islamic State (IS) militants in parallel but separate offensives in the oil-rich eastern province of Deir el-Zour.

Syrian forces are attacking the provincial capital, Deir el-Zour city, while SDF fighters are battling IS militants on the east side of the Euphrates River.

The SDF said six of its fighters were wounded in the Russian bombing of an industrial area northeast of the city, about four miles (seven kilometers) from the east bank of the Euphrates.

"At 3:30 am (0030 GMT) on September 16, 2017, our forces east of the Euphrates River were targeted by Russian and Syrian regime warplanes in the Al-Sinaaiya area," the SDF said.

"At a time when the SDF's brave forces are scoring great victories against IS in Raqqa and Deir Ezzor... some parties are trying to create obstacles to our progress," the statement said, using an alternative transliteration for Deir el-Zour.

Crowded air space

SDF strikes against IS in Deir el-Zour and in Raqqa, further up the Euphrates Valley, are backed by the US-coalition jets, while Syrian regime troops are getting air cover from Russia.

SDF fighters are not coordinating their offensive in Deir el-Zour with the Syrian government or Russia. But officials say there is a de-confliction line to prevent the two sides from clashing. That line runs from Raqqa province southeast along the Euphrates River to Deir el-Zour.

Air space over Syria has become increasingly crowded as the six-year-old civil war drags on. Warplanes from the US coalition, Russia and Syria are all carrying out airstrikes.

Confrontations between opposing warplanes have been rare, but in June a US military jet shot down a Syrian warplane believed to have attacked SDF units.

Bouthaina Shaaban, a top adviser to Syrian President Bashar al-Assad, told Al-Manar television that it was up to Russia and the US to ensure the SDF and Syrian army do not clash.

"Whether it's the SDF, Daesh, or any illegitimate foreign forces... we will work against them until all our land is liberated," she said.

"It's the role of Russia and America to see that this confrontation doesn't take place," she said.

Syria descended into civil war in 2011 shortly after protests erupted demanding the ouster of Assad. It has since morphed into a complex, multi-front war that has killed 330,000 people and displaced millions.

dj, bik/kl (AFP, AP, Reuters)