A boy who survived what activists say is a gas attack cries as he takes shelter inside a mosque in the Duma neighborhood of Damascus. Mohamed Abdullah/Reuters

An emergency meeting of the U.N. Security Council Wednesday called for a prompt investigation into an alleged deadly gas attack by Syrian President Bashar Assad's forces that activists say killed hundreds of civilians outside Damascus, the Syrian capital. If confirmed, the attack would be the worst reported use of chemical arms in the two-year-old civil war, and would apparently cross what President Barack Obama has called a "red line."

"There is a strong concern among council members about the allegations and a general sense that there must be clarity on what happened and the situation must be followed closely," Argentina's U.N. ambassador current Security Council president, Maria Cristina Perceval, told reporters after a two-hour, closed-door emergency meeting of the council.

Al Jazeera's John Terrett, reporting from New York, described the statement released by the Council as "very vague, bland and tepid."

"The Security Council is hobbled on issue of Syria; they can't agree on anything," he said.

The United States and other Western and regional countries called for U.N. chemical weapons investigators -- who arrived in Damascus just three days ago -- to be urgently dispatched to the scene of what appears to be one of the deadliest incidents of Syria's war.

But Ake Sellstrom, who is heading the U.N. team in Syria, says a formal request from a member state would have to come through the U.N. channels and Syria would need to agree in order for the team to investigate the newest accusations.

U.N. diplomats, however, said Russia and China opposed language that would have demanded a U.N. probe.

Syria's ambassador to Russia also said Wednesday that information that government troops used chemical weapons near Damascus is not true. While Russian Foreign Minister Sergei Lavrov said the reported chemical attack could be a "provocation" but called for a "fair and professional" investigation into the claims.

The White House said Wednesday it was "deeply concerned" over the reports and that it would consult with other Security Council members. It also said it had no "independent verification" about the use of chemical weapons in Syria.

"We are formally requesting that the United Nations urgently investigate this new allegation," White House spokesman Josh Earnest said. "If the Syrian government has nothing to hide," he said, it would facilitate the work of the inspectors.

In June, the United States said it had conclusive evidence that Assad's regime used chemical weapons against opposition forces -- prompting a U.S. decision to begin arming rebel groups, although that has yet to happen.

Meanwhile, the Arab League echoed the call of the Security Council, asking that U.N. weapons inspectors already in the country be allowed to visit the site of the alleged new chemical weapons attack.