(CNN) — Syrian opposition groups claimed that hundreds were killed Wednesday when the government used chemical agents in rebel strongholds in the Damascus countryside.
Videos posted online by opposition activists show lifeless bodies, most with no visible signs of injury. Many of the injured appeared to be convulsing.
CNN could not immediately verify where or when the videos were recorded.
Bashar al-Assad’s government dismissed the allegations, calling them “completely baseless.”
A senior U.S. administration official said the United States has no official confirmation chemical weapons were used in recent attacks in Syria.
“If true, it would be further evidence of unconscionable brutality by a desperate man and a desperate regime,” the official said, referring to al-Assad government.
The allegations come as a U.N. group is in the country to determine whether either side in the bloody civil war is using such chemicals.
“We are aware of the reports and we are trying to find out more,” U.N. spokesman Martin Nesirky said about the allegations.
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.
“The inspectors will not come,” said a resident who didn’t want his real name used. “If they wanted to come, they would have come a long time ago.
“The Assad regime determines where the inspectors go, and they will not let them go there. There is already a siege around Eastern Ghouta from the Assad regime.”
Dr. Abu Said at a field hospital in Sakba, east of Damascus, said the injured started streaming in shortly after predawn prayers. Of the 200, 40 were pronounced dead.
A man who referred to himself as volunteer first responder Abu Gazi said he heard what he described as rockets unlike any that had been heard before.
He went to Zamalka, the closest area to him that was hit. Over a few hours, he experienced blurry and a loss of vision and felt paralyzed. At one point, he said he was unable to hold himself up and collapsed.
Abu Gazi said he was with a doctor at a field hospital in Arbeen who reported 300 dead and 500 wounded.
The opposition Local Coordination Committees in Syria claimed more than 650 people had been killed by poisonous gases.
The symptoms, he said, included unconsciousness, foaming from their nose and mouth, constricted pupils, fast heartbeat and difficulty breathing.
The ones who died asphyxiated, he said.
A team of U.N. chemical weapons inspectors landed in Syria this week to begin probing allegations that chemical weapons have been used during the bloody civil war there. The inspectors started their work on Monday.
British foreign secretary, William Hague, called on the Syrian government to give access to the U.N. team.
“I am 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 said. “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.”
Hague said the UK will bring up the issue with the U.N. Security Council.
In denying the use of chemical weapons, a government spokesman said the reports were an “attempt to divert the UN chemical weapons investigation commission away from carrying out its duties,” SANA reported.
Syria has been embroiled in a war for more than two years, during which more than 100,000 people have been killed and millions have been displaced or become refugees in other countries, according to the United Nations.
Amid the fighting, there have been numerous allegations that chemical weapons have been used.
In June, the White House said Syrian President Bashar al-Assad’s forces had crossed a “red line” by using chemical weapons, including the nerve agent sarin gas, against rebel forces. This prompted the U.S. government to begin providing military support to opposition fighters, despite its earlier reluctance to do so.
Syria’s government, meanwhile, has claimed rebel fighters have used chemical weapons as well. That includes a March incident in Khan al-Asal in the northern province of Aleppo, according to state media.
Opposition officials have said rebels don’t have access to chemical weapons or the missiles needed to use them in an attack, while other rebel leaders said Syrian troops fired “chemical rockets” at civilians and opposition forces.
The government has agreed to arrangements “essential for cooperation to ensure the proper, safe and efficient conduct of the mission,” the secretary-general’s office said last week. Khan al-Asal will be one of the three incidents that U.N. inspectors will look into, a U.N. official said in late July.
CNN’s Arwa Damon, John King, Saad Abedine and Mohamed Aly contributed to this report.