BEIRUT (AP) — Syrian rebels accused government forces of launching toxic gas attacks on civilians in a town southwest of Aleppo on Tuesday. The government rejected the claim and accused the rebels of using chemical weapons themselves.

Rebel sources provided video of people receiving treatment who they say were among the victims of a gas attack, but the images were not conclusive and neither of the gas attack claims by the rebels or the government could be independently verified.

The accusations on both sides came amid heightened fighting around the contested northern city that killed at least 20 people, activists and government media reported.

Rescuers and doctors in rebel-held Saraqib, a town in the northwestern Idlib province, about 40 kilometers (25 miles) southwest of Aleppo, reported dozens of cases of severe breathing difficulties, saying the symptoms pointed to a chlorine gas attack.

He said first responders smelled the gas at the site of the bomb attack, which he described as a busy shopping area near an ice cream shop. Rebels and activists have reported chlorine gas attacks in the town before, but the lack of chemical labs or independent testers makes it difficult to verify these claims. The government denies that is has launched chlorine gas attacks.

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.

Fighting has intensified around Aleppo and its neighboring provinces since the government sealed off the final route into rebel-controlled neighborhoods of the city. The United Nations said the encirclement of rebel-held areas of deeply divided Aleppo traps nearly 300,000 residents, making it the largest besieged area in war-torn Syria.

Rebel fire killed at least nine civilians, including three children, in the government-held portion of Aleppo, while presumed Russian or Syrian government airstrikes killed at least 11 in Atareb, a town to Aleppo's west, activists and government media reported.

The rebels have launched a counteroffensive, clashing with government forces on a number of fronts. They detonated at least one large tunnel bomb underneath a government position on the southern outskirts of Aleppo on Tuesday evening, in a renewed attempt to break the government's siege on the city's east.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said fighters were killed on both sides in battles around the southern al-Ramouseh and al-Amerieh neighborhoods of the city, but did not have exact figures. Several rebel factions, including the Levant Conquest Front— formerly known as the Nusra Front and linked to al-Qaida — announced they had taken government positions there and killed soldiers.

The Observatory said at least 11 people, including five children, were killed when bombs were dropped in a market in the rebel-held town of Atareb. The activist-run Aleppo Media Center said the death toll was 20, posting pictures of destroyed buildings and civilians searching for survivors.

On Tuesday, residents of the rebel-held parts of Aleppo returned to burning tires, sending up big plumes of black smoke in the hope of preventing airstrikes.

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But anti-government activists and residents reported that airstrikes and unguided barrel bombs hit various parts of the city. At least one person was killed in al-Sukkari neighborhood, according to the activist-operated Local Coordination Committees. On social media, rebel fighters asked residents to stay indoors and away from front lines.

Syria's state news agency reported that four women and three children were among the nine killed when rebels shelled and fired at three front-line neighborhoods on Monday night.