UNITED NATIONS, April 8 (Xinhua) -- United Nations (UN) Secretary-General Antonio Guterres on Sunday expressed his grave concern about the alleged use of chemical weapons in Syria.

Stephane Dujarric, spokesman for Guterres, said in a statement that "the secretary-general is deeply concerned about renewed and intensive violence in Douma in Eastern Ghouta over the last 36 hours, following a period of relative calm."

"The secretary-general is particularly alarmed by allegations that chemical weapons have been used against civilian populations in Douma," said the spokesman.

"While the United Nations is not in a position to verify these reports, the secretary-general notes that any use of chemical weapons, if confirmed, is abhorrent, and requires a thorough investigation," he said.

"It is critical that civilians be protected. The secretary-general calls on all sides to ensure respect for international humanitarian and human rights law, including humanitarian access across Syria to all people in need, as per relevant Security Council resolutions," said the spokesman.

Opposition activists and local rescuers said at least 40 people, including families found in their homes and shelters in Douma, the last rebel-held area in the Eastern Ghouta countryside of Damascus, were killed in an attack on Saturday, in which Syrian forces used chlorine gas.

The Syrian government has denied the allegations, calling them fabrications.

The UN chief also showed his concern about reports of "sustained airstrikes and shelling on Douma, killing of civilians, destruction of civilian infrastructure, and attacks damaging several health facilities."

"The secretary-general calls on all parties to cease fighting and restore the calm that had been in place," the spokesman said. Guterres reiterated that "there is no military solution to the conflict."