Russia's increasing military action in Syria is forcing moderates within the opposition into the hands of extremists, the US has said.
It was making a difficult situation "more confused", the state department said as the US warned it might stop co-operating with Moscow over Syria.
But Russia accused America of trying to spare a jihadist group in its bid to oust Syrian President Bashar al-Assad.
Government forces and rebels shelled each other overnight in Aleppo.
The government is pressing on with an offensive in the country's second city, which it began last month after a truce brokered by the US and Russia broke down.
Russian Foreign Minister Sergei Lavrov said the US had broken its promise to separate the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) and other extremist groups from more moderate rebels.
State department spokesman Mark Toner said the Russian allegations were "absurd". He told reporters the US had not targeted al-Nusra for months because they had become "intermingled" with other groups and civilians.
He said a recent attack on an aid convoy, which the US blames on Russia, and renewed air strikes on the city of Aleppo were driving moderate rebel groups closer to the extremists.
"You've got a scenario now, a dynamic where, as these moderate opposition forces are under increasing pressure from the regime, that they are driven into the arms [of al-Nusra], and they have to fight side by side," he said.
Mr Lavrov accused the US of breaking its promises during an interview with Stephen Sackur on BBC World News TV, on the first anniversary of the beginning of the Russian air campaign in Syria.
"They [the US] pledged solemnly to take as a priority an obligation to separate the opposition from Nusra," he said.
"They still, in spite of many repeated promises and commitments... are not able or not willing to do this and we have more and more reasons to believe that from the very beginning the plan was to spare Nusra and to keep it just in case for Plan B or stage two when it would be time to change the regime."
The recent US-Russian cessation of hostilities deal was meant to lead to joint Russian-US air strikes on the Islamic State group and Jabhat Fateh al-Sham, which is linked to al-Qaeda.
But many of the more moderate rebel groups that the US backs have formed a strategic alliance with Jabhat Fateh al-Sham and now fight alongside it.
Aleppo's deadly week
Once Syria's commercial and industrial hub, Aleppo has been divided roughly in two since 2012, with government forces controlling the west and rebel factions the east.
According to the Syrian Observatory for Human Rights, a UK-based opposition monitoring group, new air strikes were launched after midnight on Friday.
A spokesman for the Syrian American Medical Society, a non-governmental organisation, told AFP news agency two barrel bombs had hit a hospital it supports.
The M10 hospital has been out of service since it was bombarded on Wednesday.
The UN says 400 civilians, including many children, were killed in the city this week as a result of bombardment by Russian and Syrian government forces.
Some 250,000 people are trapped in the east in appalling conditions.
Meanwhile, the Observatory said 16 civilians, including five children under the age of 18, had been killed by rebel shelling of west Aleppo on Friday.