Syrian rebels claim they have captured the entire western area of Aleppo, a northern province that has seen harsh clashes between the forces of President Bashar al-Assad and opposition fighters for months.

“We managed to liberate the western entry point to Aleppo. We achieved this victory against the Assad forces and Iranian Revolutionary Guards, along with some forces from Hezbollah,” rebels said in a video shared on the Internet on Tuesday.

“At this point, we can say that Khan al-Assal has been liberated entirely and is in our control, which means that Aleppo’s countryside is in our control.”

Khan al-Assal was the last regime bastion in the western part of Aleppo province, which lies on the Turkish border, said the Syrian Observatory for Human Rights, a UK-based NGO monitoring developments in Syria through intelligence from activists in the country.

The rebels also seized the villages of Obeida and Hajireh, southeast of Aleppo city on Monday, the group said.

The Syrian government and the opposition have accused each other of being behind a chemical missile attack that killed at least 25 people in Khan al-Assal.

On Tuesday, Robert Serry, a UN envoy on the Middle East peace process, said that his organisation had been told of 13 alleged chemical weapons attacks in Syria.

The allegations came as the head of a UN investigation into the use of chemical arms in Syria, Ake Sellstrom, and the UN disarmament chief, Angela Kane, arrived in Syria.

The two officials were expected to go to Damascus on Wednesday to start talks with Assad's government on securing access to sites in Syria where the arms are said to have been used.