BEIRUT – The United Nations Children’s Fund said 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 neighborhoods under government siege.

The UNICEF statement came as Russia said it has informed the United States that insurgents attacked a government-held neighborhood 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 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 children make up a third of the 300,000 residents trapped in rebel-held besieged neighborhoods.

In the western, government-controlled areas, UNICEF said 25,000 people have been displaced and are taking shelter from intense fighting in mosques, on university campuses and in 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 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 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 center.

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 facilities in that region since the Syrian conflict began.

The group said it verified each attack. It said all six facilities hit from July 23 to July 31 were major hospitals, including a referral hospital just outside opposition-held eastern Aleppo and a pediatric clinic inside the city where four infants died after their oxygen supply was cut.