The unwillingness of the United States to work with Russia in Syria contributed to the loss of the ancient city of Palmyra to the Islamic State group, a Moscow spokesman has said.
Dmitry Peskov told journalists that "the US does not want to cooperate", adding that "cooperation would have probably allowed us to more effectively avoid such attacks from terrorists".
Syrian troops aided by Russian airstrikes drove IS from Palmyra in March, but the militants stormed back into the city over the weekend - forcing the army to retreat despite a wave of Russian airstrikes.
The Syrian government and Russia had been focused on Aleppo, where they are close to defeating rebels that have held the eastern half of the northern city for four years.
Mr Peskov said that many of the IS fighters involved in the Palmyra assault had recently left the Iraqi city of Mosul, where US-backed Iraqi forces have been waging a two-month-old offensive against the militants.