RUSSIAN WARNING

Russia, like Iran an ally of President Bashar al-Assad, issued a warning of its own to the United States in response to the downing of the Syrian jet, saying on Monday it would view as targets any planes flying west of the Euphrates River, though it stopped short of saying it would shoot any down.

Still, the Pentagon said it had not seen hostile action by Moscow.

"Public statements aside, we have not seen the Russians do any actions that cause us concern. We continue to operate, making some adjustments for prudent measures," Davis said.

Meanwhile, the US and Russian militaries swapped accusations about an unsafe intercept involving a US spy plane and a Russian fighter jet over the Baltic Sea.

In Syria's tangled conflict, Washington backs a coalition of rebel forces fighting both President Bashar al-Assad and Islamist militants, while Assad is backed by Russia, Iran and Shi'ite militia.

The US military has repeatedly warned forces fighting on Assad's side to stay away from a "deconfliction zone", agreed with Russia, near a garrison used by US special forces and US-backed militia around Al Tanf.

On several occasions in recent weeks, warplanes of the US-led coalition have also struck pro-government forces to prevent them advancing from the Al Tanf garrison in southeastern Syria at a spot where the country's borders join Iraq and Jordan.

Washington also described those strikes as self-defense.

The competition between the Syrian army and the militias and US-backed Free Syrian Army (FSA) rebels has stepped up in the Badia desert that stretches to the Iraqi border after Islamic State abandoned large swathes of territory as it defends Raqqa and Deir Zor.

The Syrian army has been able to make rapid advances allowing them to reach the border for the first time in years.

Reuters