Longtime Syria ally Russia has provided air cover for pro-government forces for nearly a year, including in Aleppo.
The UN said Russia was considering expanding three-hour pauses in fighting every morning to bring in desperately needed aid.
"Any pause obviously should always be seen and looked at with great interest, because a pause means no fighting, but three hours is not enough," said envoy Staffan de Mistura.
Russia was meanwhile offered the possibility of joint operations against IS by Turkey, which has backed rebels against President Bashar al-Assad.
The offer came one day after crucial talks between President Vladimir Putin and Turkish counterpart Recep Tayyip Erdogan.
While it is unclear how possible cooperation between Moscow and Ankara -- which support opposing sides -- would work, the US tentatively welcomed the idea.
Working against IS "is a priority for all of us", State Department spokeswoman Elizabeth Trudeau said.
"If this is truly a step in that direction, we would welcome that."
- 'We do not need tears' -
Jan Egeland, who heads the UN-backed Syria humanitarian taskforce, said he was "hopeful" talks with Russia could lead to aid entering the city.
But rebels and regime forces clashed in southern Aleppo, including during the period when the pause was meant to take hold, said the Syrian Observatory for Human Rights.
Rebels and jihadists broke a three-week government siege of Aleppo's east on Saturday, opening a new route for goods through the southern outskirts.
An AFP correspondent in the east said trucks carrying food were unable to enter the city because of intense bombardment.
But the Red Crescent brought diesel to a pumping station and as a result water was restored to some areas in east and west Aleppo after six days.
An estimated 1.5 million people live in Aleppo, including about 250,000 in rebel-held districts.
State media said army troops seized territory south of Aleppo, and that rebel fire killed four civilians in a government-held district.
But it made no mention of the "humanitarian windows" announced by Russia.
Fifteen of the only remaining doctors in eastern Aleppo implored US President Barack Obama to protect civilians from atrocities.
"Unless a permanent lifeline to Aleppo is opened it will be only a matter of time until we are again surrounded by regime troops, hunger takes hold and hospitals' supplies run completely dry," they wrote.
"We do not need tears or sympathy or even prayers, we need your action. Prove that you are the friend of Syrians."
Human Rights Watch said it had documented six strikes by regime or Russian warplanes on health facilities in the north that killed 17 people in the past two weeks.
"With heavy bombing continuing relentlessly in Aleppo especially, hospitals and clinics need to be treated as the sacred life-saving places they are, not as additional bombing targets," said HRW's Nadim Houry.
- Concerns over chemical weapons -
French Foreign Minister Jean-Marc Ayrault said he was "concerned by reports of a new chemical attack... that is said to have claimed four lives and left dozens injured."
Washington also expressed concern over the reports, which it said would be in violation of a 2013 UN resolution to dismantle the Syrian government's chemical weapons arsenal.
Activists accused government forces on Wednesday of carrying out an attack using chlorine gas on a rebel-held residential neighbourhood.
De Mistura said he could not verify the reports.
Further east, Russian raids hit the IS stronghold of Raqa, killing at least 24 civilians and wounding 70 people, said the Observatory.
Russia said the raids destroyed a "chemical weapons factory" as well as a weapons storage facility and IS training camp.
It said the jihadists suffered "significant" material damage and a large number had been killed.
Syria's conflict erupted in March 2011 and has since killed more than 290,000 people and drawn in world powers on all sides.