The White House said the parties are intentionally targeting civilians to bomb them “into submission.
“People of good conscience around the world should speak up and are speaking up,” spokesman Josh Earnest told reporters.
President Barack Obama is “deeply concerned” about the situation, Earnest said.
Aleppo is being pummeled in a joint operation by Russian and Syrian forces that began four days ago, after a seven-day truce failed to stymy the violence and initiate nationwide aid deliveries to besieged areas.
At least 85 people were killed and 178 others injured Sunday in Syrian and Russian airstrikes on residential areas in opposition-held parts of Aleppo.
UN Secretary General Ban Ki-moon said that day that he is appalled by the “military escalation” in Aleppo, and that the use of “indiscriminate weapons in densely populated areas may amount to war crimes.”
Russia's support for the Syrian government offensive is only playing into the hands of extremist groups, Earnest said. "Russia is going to have to deal with the fallout because their actions are fueling extremism, not just in Syria and not just in the region, but around the world, including in Russia."
Syria’s pre-war largest city has been divided into two since 2012 when rebel forces ousted government troops. Assad’s forces continue to hold the west, while rebels hold the east.
The Syrian Center for Policy Research, a Beirut-based NGO, has put the death toll from Syria's six-year-old conflict at more than 470,000.