SYRIAN GOVERNMENT ACCUSED OF USING CHEMICAL WEAPON.

BEIRUT — Syrian activists accused the government Wednesday of launching a massive chemical weapons attack that killed scores of people in the Damascus suburbs and left makeshift hospitals packed with victims gasping for breath.

The death toll from the alleged attack — which the government strongly denied — varied vastly. The Britain-based Syrian Observatory for Human Rights said at least 100 people were confirmed killed, but that the number was likely to rise, while the Syrian Opposition Coalition claimed that at least 1,300 died. The opposition Damascus Media Office also put the toll at more than 1,000.

The United States and Britain separately expressed deep concern, demanded an urgent and unfettered investigation by a United Nations team already in Syria and called for emergency consultations in the U.N. Security Council. They strongly condemned any use of chemical weapons and said anyone responsible must be held accountable.

“The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons, near Damascus earlier today,” White House deputy press secretary Josh Earnest said in a statement. “We are working urgently to gather additional information.”

Earnest said the U.N. investigators now in Syria “must have immediate access to witnesses and affected individuals, and have the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government.” He said the United States also wants the Security Council to call for the government of Syrian President Bashar al-Assad to provide such access.

U.N. Secretary General Ban Ki-moon is “shocked” by the reports, which the U.N. team in Damascus is discussing with Syrian authorities, according to a statement issued by his office.

Dozens of graphic videos were posted online showing the alleged victims. Many appeared to be children who were vomiting, convulsing and struggling to breathe.

In one video, men sprawled on the floor of a makeshift hospital are hosed down with water. Another shows a listless child being treated with a hand-held respirator. Others show victims gasping for breath.

It was not immediately possible to confirm whether the people depicted in the videos were exposed to chemical weapons. Syria’s state news agency called the reports “completely baseless.”

The allegations surfaced as a U.N. team of chemical weapons experts was visiting Syria to investigate past reports of the use of toxic gases in the civil conflict that has raged for 2 1 / 2years. The reported cases include an alleged attack in March in the village of Khan al-Assal, near the northern city of Aleppo, that left 19 people dead.

Wednesday’s alleged attack was in Ghouta, an area just outside the capital that is known for its support of the Syrian rebels. Activists said rockets primed with chemical weapons struck at least seven locations. A bright white light lit up the sky in a video purporting to show one of the strikes.