An air strike in the rebel-held part of Syria's contested city of Aleppo has killed 10 civilians, activists said, an attack that hit in the same neighbourhood where a suspected chlorine gas attack happened the day before.
Meanwhile, the UN aid agency said the fighting in Syria's central Hama province has displaced some 100,000 people over eight days between late August and early September.
Ultraconservative Islamic insurgents last week advanced in Hama, prompting fierce clashes with government forces.
In Wednesday's airstrike, the Britain-based Syrian Observatory for Human Rights said at least one child was among the victims of what was a presumed to have been a Russian or Syrian government attack on the al-Sukkari neighbourhood in Aleppo.
The long-suffering northern city is one of the focal points of the grinding Syrian civil war, now in its sixth year, with rebels and pro-government forces trading indiscriminate fire across populated neighbourhoods. The Russians and the Syrian government are the only two operating in the skies over the city.
The Aleppo branch of the Syrian Civil Defence search and rescue organisation put the initial casualty toll at 20 dead and more than 40 wounded, but conflicting counts are common in the aftermath of air strikes.
Medical workers in the city have said the opposition-controlled neighbourhood was hit with chlorine gas on Tuesday, though the report could not be independently verified. They said they treated at least 70 people for breathing difficulties. A 13-year-old girl and a 29-year-old man died from further complications on Wednesday.
In Syria's central Hama province, the insurgents earlier this month surprised government troops, dislodging them from areas around the provincial capital, also called Hama, including a military base and towns and villages near the highway leading to Damascus.
The offensive, led by an ultraconservative Islamic group, Jund al-Aqsa, and also involving several factions from the Western-backed Free Syrian Army, drew an intense government bombing campaign that has killed dozens.
The fighting and the aerial bombardment sent tens of thousands of people fleeing for safety, creating the latest wave of displacement, part of a pattern that has left nearly half of the Syrian population displaced since the war began in 2011.
In a "flash update " on Tuesday, the Office for the Co-ordination of Humanitarian Affairs (Ocha) said figures from a camp coordination group show nearly half of the displaced from Hama have arrived in the neighbouring rebel-held Idlib province.
Others fled toward government-controlled Hama city, where four mosques were converted into temporary shelters, Ocha said. Dozens of schools in rural areas of Hama province were also turned into shelters.
A shortage of shelter space means many displaced families are sleeping outdoors in parks in Idlib, the UN agency said.
Dozens of children are suffering after suspected chlorine bombs hit Aleppo. pic.twitter.com/FBDHWWKDGA— AJ+ (@ajplus) September 7, 2016
Most of those fleeing left towns and villages in government areas as the rebels advanced, fearing a violent government response to the insurgent offensive, according to Ahmad al-Ahmad, an activist from Hama. "Wherever the regime is driven out of an area, it ends up destroying it," he said in a text message to The Associated Press.
In at least one air strike last week, government warplanes struck a van carrying displaced people fleeing Suran, a town north of Hama city, activists said. The government at the time said it targeted "terrorists."
Ocha said the United Nations has sent an "inter-agency convoy with life-saving supplies to Hama" and was evaluating the humanitarian situation.
An estimated 11 million Syrians have fled their homes since the war broke out in 2011. Of those, 4.8 million are refugees outside of Syria, with nearly seven million displaced inside the country.
In London on Wednesday, Syrian opposition leaders unveiled a plan for a political transition designed to bring an end to the war. It called for the departure of President Bashar Assad after six months and for elections to be held after two years.
The High Negotiations Committee (HNC) envisaged a three-phase plan, beginning with six months of negotiations with Assad's government to develop a signed agreement on the "basic principles" of the transition process.
This would be followed by the establishment of a transitional government body and the departure of Assad "and his clique," according to HNC chief Riad Hijab.
The HNC called for UN-supervised elections to be held 18 months thereafter. Mr Hijab conceded there were formidable obstacles hindering the implementation of this plan.
In London, Foreign Secretary Boris Johnson endorsed the opposition plan, saying in a column in The Times that Mr Assad can have no part in a future government in Syria and that the Syrian president bears "overwhelming responsibility" for the massive loss of life in the conflict.
He also said that if Russian and American negotiators can create a ceasefire there could be a resumption of talks in Geneva aimed at ending the war. The relatively new Foreign Secretary was to host a Friends of Syria meeting in London later on Wednesday.