An activist in a gas mask on Aug. 22, 2013, in Zamalka, where activists say chemical weapons were used. Bassam Khabieh/Reuters

Russia -- one of the most tenacious allies of the Syrian regime -- has called on the the government of President Bashar al-Assad to allow a mission of United Nations inspectors to investigate alleged chemical attacks in the suburbs of Damascus and guarantee safe passage for U.N. workers entering the country, according to Reuters.

Russia has also called on forces opposed to Assad's government to guarantee the investigators' safety. Russia's appeal to Syria comes after months of warnings from Moscow against foreign intervention in the two-year civil war that has left more than 100,000 people dead and displaced nearly 2 million Syrians, including 1 million children.

Video footage that has not been independently verified by Al Jazeera shows what appears to be the remnants of a chemical weapons attack.

U.N. Secretary-General Ban Ki-moon asked the Syrian government to allow U.N. inspectors to investigate the allegations "without delay."

Ban said the use of chemical weapons in Syria would constitute a "crime against humanity" and a violation of international law. He also warned Assad's government of "serious consequences" if their use is proved.

Opposition groups alleged Assad's government on Wednesday fired rockets with chemical warheads into the Damascus suburbs of Arbeen, Zamalka and Ein Tarma. The attacks reportedly left hundreds dead.

"This is a grave challenge to the entire international community -- and to our common humanity, especially considering it occurred when the United Nations expert mission is in the country," Ban said.

Ban on Thursday instructed Angela Kane, the U.N.'s high representative on disarmament affairs, to travel to Damascus, according to a statement.

Meanwhile, activists said government warplanes on Thursday again hit areas where the alleged chemical attack took place.

Syria's government, which has repeatedly denied the use of chemical weapons, offered no public response to the U.N. calls for its team to inspect the site of the attack.