WASHINGTON (AFP) - The Pentagon held talks with Moscow officials Thursday to try to avoid mishaps between the two military powers, though it wasn't clear how fruitful the effort was amid a second day of Russian bombing in Syria.

US military officials were furious Wednesday after Russia only gave them an hour's vague "heads-up" it was about to begin bombing. The warning didn't specify when or where the strikes would occur, only that coalition planes should avoid the area.

With a US-led coalition carrying out near-daily plane and drone strikes in Syria, the new reality of Russia flying sorties in the same air space has left the Pentagon worried about planes crossing paths and sparking a major international incident.

Pentagon press secretary Peter Cook said Defence Department officials spoke with Russian counterparts for about an hour via video in what he said was a "cordial and professional" exchange.

He gave few details but said officials discussed which international frequencies could be used if a pilot was in distress and what language aircrews should communicate with each other in.

"We made crystal clear that at a minimum the priority here should be the safe operation of the aircrews over Syria," Cook said. No follow-up calls had been scheduled yet, he added.

The United States has repeatedly stressed the urgent need for Russia to communicate with it about when and where it plans to fly its fighter jets and bombers. In military jargon, such discussions are known as "deconfliction."

Russia on Wednesday launched its first air strikes in Syria, marking its explosive arrival in the 4.5-year-old conflict that has claimed some 250,000 lives.

Strikes continued Thursday with Russian warplanes hitting opponents of Syrian President Bashar al-Assad.

The Russians currently have at least 32 warplanes deployed in Syria, US officials say.