(We most recently updated this post at 9:10 a.m. ET.)

"Two Syrian pro-opposition groups are claiming that dozens of people were killed Wednesday in a poisonous gas attack near Damascus," NPR's Jean Cochran reported earlier this morning on our Newscast. The groups are blaming the attack on government forces, she said.

"The reports have not been independently confirmed," Jean added, and President Bashar Assad's regime says the claims are "baseless."

Reuters began Wednesday leading its report this way: "Syrian activists accused President Bashar al-Assad's forces of launching a nerve gas attack that killed at least 213 people on Wednesday, in what would, if confirmed, be by far the worst reported use of poison gas in the two-year-old civil war." Shortly after 9 a.m. ET, the wire service said activists were claiming that nearly 500 people had died. "If confirmed," Reuters added, it would be "by far the worst reported use of chemical arms in the two-year-old civil war."

This news comes as a U.N. team is in Syria to investigate earlier alleged uses of chemical weapons.

The BBC's Naomi Grimley adds that opposition groups have released "distressing pictures [that] show children lying limp in the arms of adults" and adults being hosed down, allegedly to wash away chemicals. But, she says, those images haven't been independently verified.

According to CNN, "the explosions took place in eastern and western Ghouta, rebel strongholds that the regime has for more than a year been desperately trying to take back. They don't want rebels pushing into the capital."

British Foreign Secretary William Hague says in a statement released by his office that he is "deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus."

Hague also says that:

"These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria. Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account. I call on the Syrian Government to allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use. The U.K. will be raising this incident at the UN Security Council."

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