Army Colonel Steve Warren told a Pentagon briefing by video from Iraq that the coalition had seen eight to 10 Russian aircraft leave Syria and that a number of small ground units near the airport were packing up to head home, Reuters reported.

At the same time, he added, Russian planes had flown sorties since the withdrawal announcement, although they had not conducted air strikes. And Russian artillery units supporting Syrian troops attacking Islamic State near Palmyra were still in the field, he added.

"It's difficult to know what the Russians' intentions are," Warren said. "We were not expecting this announcement in the first place, so as we compare their words to their actions, we'll have to wait to see what develops."

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Western diplomats have speculated that Russian President Vladimir Putin was trying to pressure Syrian President Bashar al-Assad to accept a political settlement of the five-year-old war.

Putin, in a surprise move on Monday, announced the main part of Russian forces in Syria would start to withdraw. The announcement came as the parties to the conflict resumed peace and a cessation of hostilities significantly reduced violence.

Warren said the coalition had been "working diligently" to assess what the Russians' intentions were and what impact the withdrawal would have on fighting on the ground.

"It's difficult to tell. It's been less than 48 hours so far," he said. "There's been a relatively minor withdrawal of air combat power. There has been an equally minor withdrawal of ground forces (but) ... we haven't seen a significant reduction frankly in their combat power."

Warren said there had been a "fairly notable reduction" in the fighting between the forces of Syrian President Bashar al-Assad and his more moderate opposition as a result of the cessation of hostilities agreed as peace talks began.

But Assad's forces were continuing their offensive against Islamic State militants in Palmyra, and "our general assessment ... is that the regime could continue to push if they received Russian air support," he said.

"We're going to continue to focus on fighting ISIL (Islamic State) and watching what the Russians do so that we can decide how to proceed," Warren said.