The Kremlin on Monday deplored the lack of cooperation with the United States in Palmyra after Islamic State jihadists re-entered the ancient Syrian city over the weekend.

"We regret that we have yet to completely neutralise their offensive," Kremlin spokesman Dmitry Peskov said of the fighters' return to the fabled city after an eight-month absence.

"We also regret that there still is a lack of coordinated action and real cooperation with other states -- with the United States first and foremost -- that do not want to cooperate, and this cooperation could allow us to avoid such attacks by terrorists."

Peskov added that jihadists from neighbouring Iraq, where a Western coalition is supporting the Iraqi military's efforts to retake the city of Mosul from IS, had been flooding into Syria.

Russia's Foreign Minister Sergei Lavrov later suggested that the movement of IS fighters fleeing an Iraqi assault on Mosul to Syria and the offensive on Palmyra might be part of an orchestrated plan to ease pressure on rebel groups in the second city of Aleppo.

"It makes me think, and I hope I am wrong, that it is all orchestrated in order to give a respite to the bandits still in eastern Aleppo," Lavrov said.

Pentagon officials hit back at Moscow's claims, saying Russia had taken its eye off Palmyra while it focused on bombing Aleppo.

"It appears that Russia has failed to sustain the singular gain against (IS) they had achieved since their military intervention on behalf of (President Bashar al-Assad) regime," said Major Adrian Rankine-Galloway, a Pentagon spokesman.

"This is a further demonstration of why, as we have long advocated, Russia must change their focus."

Russia's defence ministry said Sunday that its war planes had carried out more than 60 overnight strikes on Palmyra, claiming to have "thwarted all terrorist attacks" on the city.

Russia has carried out a bombing campaign in Syria in support of Assad since September 2015.

Syrian troops backed by Russian air strikes and special forces on the ground recaptured the UNESCO world heritage site from IS fighters in March, delivering a major propaganda coup for both Damascus and Moscow.

Regime forces are currently focused on a major offensive fighting other insurgent groups in the second city of Aleppo that has seen them seize back most of the rebel-held stronghold.

The deadly war in Syria has killed more than 300,000 people since it started in March 2011 with a wave of anti-government protests.