Beirut - The United Nations Children's Fund (Unicef) said on Wednesday it is "extremely" concerned for the safety and well-being of children caught up in the violence engulfing the northern Syrian city of Aleppo, including the rebel-held eastern neighbourhoods under government siege.The Unicef statement came as Russia said it has informed the United States that insurgents attacked a government-held neighbourhood of Aleppo using toxic gas, killing at least seven people.On Tuesday, Syrian rebels accused the government of using poison gas against civilians in the north. That accusation was closely followed by a report on Syrian state media accusing the rebels of carrying out a gas attack in Aleppo. None of the claims could be independently verified.Aleppo witnessed intense fighting on Wednesday as insurgents tried for the third day to break the government siege imposed on opposition-held parts of the city since mid-July.Unicef's regional director, Saad Houry, called for unhindered humanitarian access to the divided city and for children to be protected. Unicef said that children make up a third of the 300 000 residents trapped in rebel-held besieged neighbourhoods.In the western, government-controlled areas, Unicef said 25 000 people have been displaced and are taking shelter from intense fighting in mosques, university campuses and public gardens.Opposition monitoring groups reported intense airstrikes and shelling on Aleppo and its outskirts. State media said government forces repelled an attack by militants aiming to break the siege on several fronts.A reporter for the Beirut-based Pan Arab Al-Mayadeen TV, Rida al-Basha, who is on the government side of Aleppo, said the Syrian army regained control of two of the three villages it lost near Aleppo earlier this week.Opposition activists in Aleppo said government forces struck several makeshift hospitals in the city, Syria's largest and once commercial centre.Airstrikes on hospitals The New York-based Physicians for Human Rights said in a statement Wednesday that over the past week, Syrian government forces launched deadly airstrikes against six hospitals in and around Aleppo. It said the attacks were the worst week for attacks on medical facilities in that region since the Syrian conflict began in 2011.The group said it verified each attack. It said all six facilities hit between July 23 and July 31 were major hospitals in Aleppo governorate, including a referral hospital just outside opposition-held eastern Aleppo and a paediatric clinic inside the city where four infants died after their oxygen supply was cut."Since June, we've seen increasing reports of attacks on civilians in Aleppo and strikes on the region's remaining medical infrastructure. Each of these assaults constitutes a war crime," said Widney Brown, PHR's director of programmes. "Destroying hospitals is tantamount to signing thousands of death warrants for people now stranded in eastern Aleppo."The Aleppo Media Centre said among those killed on Wednesday in Aleppo was media activist Ahmad abu al-Baraa. It said he was killed while covering the offensive aiming to break the siege. State TV reported that insurgents shelled government-held parts of the city, killing seven and wounding 41.The latest round of violence came a day after Syrian rebels accused government forces of launching toxic gas attacks on civilians in the town of Saraqeb, southwest of Aleppo. The government rejected the claim and accused the rebels of using chemical weapons themselves in Aleppo.Syrian doctor Ibrahim al-Assad said he treated 16 of the 29 cases brought to his hospital in the rebel-held part of Syria on Monday night. Syrian state media later reported that five people had died and eight others had experienced breathing difficulties after artillery shells laced with toxic gases landed on the old city of Aleppo. It said the shells were launched by rebels.Toxic substancesThe head of the Russian military's Reconciliation Centre at the Russian base in Syria, Lieutenant General Sergei Chvarkov, said in a statement on Wednesday that the rebels struck a residential area in Aleppo with toxic substances, killing 7 and injuring 23. He said the Russian military had informed the US about the incident.The Organisation for the Prohibition of Chemical Weapons, OPCW, said in a statement that recent media reports that highlighted the possible use of chemical weapons in Syria are of great concern. It said the agency continues to examine any credible reports it receives, including pertinent information that might be shared by state parties to the Chemical Weapons Convention.Near the capital, Damascus, government aircraft dropped barrel bombs on the rebel-held suburb of Daraya, opposition activists said. A video posted online showed a large, fuel-rich explosion.The video appeared genuine and corresponded to other AP reporting of the events.Also on Wednesday, the Britain-based Syrian Observatory for Human Rights said that a British citizen and a Slovenian national were killed while fighting with the main Kurdish force battling the Islamic State group was killed last month in the former ISIS-stronghold of Manbij near the Aleppo.The People's Protection Unit, or YPG, posted on its Facebook page photographs of the British man and identified him as Dean Carl Evans, saying he was killed on July 21. It said Martin Gruden of Slovenia was killed on July 27, in the same town.Slovenia's Foreign Ministry confirmed Gruden's death, adding that authorities are now working on bringing his remains back to Slovenia.Dozens of western fighters have joined the YPG to fight ISIS. Last month, a 24-year-old Colorado man who joined the YPG was killed in combat in Syria, his mother Susan Shirley said. She said the US Consulate in Turkey called her to tell her that Levi Shirley was killed on July 14.