The Syrian government has agreed to suspend its aerial strikes and shelling of the northern rebel-held city of Aleppo for six weeks so that a ceasefire proposed by the United Nations can be tested, the U.N. Special Envoy to Syria reportedly said on Tuesday. The announcement was made by Staffan de Mistura during a briefing at a closed session of the Security Council.

The news of the proposed truce came amid a fresh offensive launched by President Bashar Assad’s forces on Aleppo, aimed at cutting off the rebels' supply lines and renewing the Syrian government’s push to encircle and besiege opposition-held areas of the city.

“The government of Syria has indicated to me its willingness to halt all types of aerial bombing and artillery shelling for a period of six weeks, all over the city of Aleppo,” de Mistura reportedly said. However, he added that so far, no date for the beginning of the truce had been agreed upon. He also said that he would return to Syria "as soon as possible" to assess whether a temporary ceasefire can be brought into effect.

“I have no illusions because, based on past experiences, this will be a difficult mission to be achieved,” de Mistura said, according to The Associated Press. “Facts on the ground will prove if the freeze holds and can be replicated elsewhere.”

Aleppo-based opposition activists expressed concerns that a truce could be exploited by the government to gather its forces to fight elsewhere. They also raised doubts over how effective a ceasefire would be as militants of the Islamic State group are also active in the area, Al Jazeera reported. The Syrian National Coalition, which is backed by Western nations, including the U.S., said that it is awaiting a detailed proposal from de Mistura.

“The Assad regime’s compliance with any such proposal will be judged by actions, not words,” Najib Ghadbian, the coalition’s special representative to the U.N., reportedly said. “And thus far his actions have been only brutality and terror.”

Syria’s ambassador to the U.N., Bashar Ja’afari, has so far not commented on reports of the proposed truce.