It comes after the United States threatened to halt its diplomacy with Russia on Syria.
Following the collapse of a week-long ceasefire in Syria, there has been an escalation in attacks on rebel-held parts of Aleppo, formerly the country's largest city.
The attacks include strikes on two hospitals in the opposition-controlled east.
The medical charity Médecins Sans Frontières says the two attacks have killed at least two patients and injured two medics.
It is unclear who carried out the bombings, but United Nations Secretary-General Ban Ki-moon has denounced them.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"Let us be clear. Those using ever more destructive weapons know exactly what they are doing. They know they are committing war crimes. Imagine the destruction. People with limbs blown off. Children in terrible pain with no relief, infected, suffering, dying, with nowhere to go and no end in sight. Imagine a slaughterhouse. This is worse. Even a slaughterhouse is more humane."
Dr Abd Arrahman Alomar is a doctor with the Syrian American Medical Society.
He says, following the attacks, only one trauma hospital is left in east Aleppo.
"M2, it maybe function again after two days, after fixing the damage, but, regarding M10, the damages are so huge, so we didn't know how many days it will take to fix the damage. And, by the way, M10 is the biggest trauma hospital in eastern Aleppo city, and those two hospitals are two of the only three trauma hospitals in Aleppo city. Now, Aleppo city is served, eastern Aleppo city is served, by only (one) trauma centre. This is M1 hospital."
The week-old assault has already killed hundreds of people, with bunker busting bombs bringing down buildings on residents huddled inside.
Only about 30 doctors are believed left inside the besieged zone, coping with hundreds of wounded each day.
The offensive by the government, backed by Russia, is an attempt to drive out the rebels and retake control of Aleppo.
US State Department spokesman John Kirby says the United States may suspend its engagement with Russia if the attacks do not end.
"We haven't taken those steps yet, but the message to the Foreign Minister was that we're perfectly willing and able to move forward on those kinds of steps, steps that would end up in the suspension of US-Russia bilateral engagement on Syria unless we see some significant steps taken by Russia in the very near future to show that they mean what they say when they say they support a cessation of hostilities and a resumption of political talks."
Russia says it is willing to continue negotiations with the United States over finding a political solution for the war.
Lieutenant-General Viktor Poznikhir says Russia's foreign minister has held talks with his US counterpart.
"We plan to send Russian experts to Geneva soon to resume talks with the US side to find ways to normalise the situation in Aleppo, and in Syria in general. We count on our American partners to be ready to work together."
