Dozens of people are reported to have been killed in fierce clashes between rebel groups and government forces in the divided Syrian city of Aleppo.
The Syrian Observatory for Human Rights said rebels advanced into government-held western districts overnight but were pushed back by Wednesday morning.
The battle was the most intense in Aleppo for more than a year, it added.
Later, the US said an agreement has been reached with Russia to extend a truce in Syria to include Aleppo.
The Syrian military confirmed it would observe a 48-hour ceasefire in the city.
A surge in fighting in Aleppo in the past two weeks has killed almost 300 people.
The nationwide cessation of hostilities brokered in late February by the US and Russia, which back opposing sides in the war, is close to collapse.
A coalition of rebel groups fighting under the name "Fatah Halab" (Aleppo Conquest) launched the assault on the government's defensive lines in the west of the city on Tuesday by detonating a tunnel bomb, the AFP news agency reported.
Intense gun battles, air strikes and artillery attacks went on through Tuesday night. Intermittent clashes continued on Wednesday.
Activists also reported that government warplanes carried out more than 20 air strikes in the rebel-held eastern Ghouta outside Damascus, after a freeze in fighting declared by the government around the capital expired.
On Wednesday, the US State Department announced that an extension to the cessation of hostilities had been agreed with Russia on Tuesday.
"Since this went into effect today at 00:01 (local time; 21:01 GMT on Tuesday) in Damascus, we have seen an overall decrease in violence in these areas, even though there have been reports of continued fighting in some locations," a US statement said.
Al-Nusra, an al-Qaeda affiliate that is allied to a number of rebel groups, is excluded from the cessation of hostilities.
The government and its ally Russia say only al-Nusra positions in Aleppo are being targeted, but the opposition and the US accuse them of indiscriminately attacking civilians and rebels abiding by the cessation of hostilities.
UN humanitarian adviser Jan Egeland complained later on Wednesday that the government was still refusing to allow aid deliveries to hundreds of thousands of people in besieged areas of Syria, including rebel-held eastern Aleppo.
"It is a disgrace to see that while the population of Aleppo is bleeding, their options to flee have never been more difficult than now," he told reporters in Geneva.
French Foreign Minister Jean-Marc Ayrault asserted that the Syrian government bore full responsibility for the bloodshed in Aleppo.
"What is happening in Aleppo is a tragedy that requires a stronger reaction," he said at the start of talks in Berlin with his German counterpart, Frank-Walter Steinmeier, UN envoy Staffan de Mistura and Syrian opposition leader Riad Hijab.
Russia meanwhile said it had withdrawn about 30 aircraft from its airbase in Syria, including all of the Su-25 ground attack planes stationed there.
The Russian military began the withdrawal of most of its forces from Syria in March, six months after launching an air campaign to bolster President Bashar al-Assad.