WASHINGTON — Russia says its bombing of schools, hospitals and civilians in the Syrian city of Aleppo is no different than the U.S.-backed offensive on the Iraqi city of Mosul, a charge U.S. officials fiercely deny.

The tit-for-tat is the latest sign of the downward spiral of U.S.-Russian relations over conflicts in Syria, Ukraine and cyberspace.

U.S. officials bristle at the comparison between the Russian-backed siege of Aleppo and the U.S.-backed attempt to free Mosul from Islamic State.

They are quick to point out major differences — even as some similarities are obvious.

Moscow is backing Syrian President Bashar Assad’s attempts to crush what both call terrorist groups in Aleppo, the largest city still partly under rebel control in Syria’s civil war.

Washington and its allies are backing an Iraqi-led ground offensive aimed at dislodging Islamic State from Mosul, the largest city under that group’s control.

An estimated 275,000 civilians are believed to be trapped in eastern Aleppo. An estimated 1 million Iraqis may be stuck in Mosul. Militants have blocked their escape in both cities.

But unlike in Aleppo, where Syrian and Russian attacks have caused heavy civilian casualties over the last year, the Pentagon and the United Nations said they have no confirmed civilian casualties from coalition attacks in the Mosul offensive that began two weeks ago.

U.S. officials say Russia refuses to distinguish among the insurgents in Aleppo. They say some — such as the Syrian branch of al-Qaida once known as the Nusra Front — are legitimate targets.

But fighting along with them, the U.S. contends, are more moderate rebels who oppose Assad’s continued rule and should be supported, not wiped out.