(CNN) -- Syrian opposition groups claimed that hundreds were killed Wednesday when the government used chemical agents in rebel strongholds in the Damascus countryside.
That death toll could be as high as 1,300 people, George Sabra, the head of the Syrian National Council, said at a news conference. The council is an umbrella group that includes those trying to oust President Bashar al-Assad since the spring of 2011.
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, not could CNN verify the number killed or injured.
Al-Assad's government, through state media, dismissed allegations that chemicals have been used, calling them "completely baseless."
A senior U.S. administration official said the United States had no official confirmation that 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.
The allegations come as a U.N. group is in the country to determine whether either side in the bloody civil war is using 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 been desperately trying to take back for more than a year. They don't want rebels pushing into the capital.
"The inspectors will not come," said a resident who didn't want his 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."
Hundreds reported dead
Dr. Abu Said, at a field hospital in Sakba, east of Damascus, said the injured started streaming in shortly after predawn prayers. Forty of the 200 people brought in were pronounced dead.
A man who referred to himself as volunteer first responder Abu Gazi said he heard rockets unlike any that he'd heard before.
He went to Zamalka, the closest area to him that was hit. Over a few hours, his vision blurred, and he lost vision and felt paralyzed. At one point, he said, he collapsed.
Abu Gazi said he was with a doctor at a field hospital in Arbeen who reported 300 people dead and 500 wounded.
The opposition Local Coordination Committees in Syria claimed that poisonous gases killed more than 650 people.
The symptoms, he said, included unconsciousness, foaming from the nose and mouth, constricted pupils, fast heartbeat and difficulty breathing.
The ones who died asphyxiated, he said.
Countries express concern
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 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," he 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.
The French Foreign Ministry said it didn't have independent confirmation that an attack took place as rebels claim, but it said those responsible for the alleged chemical weapons attack "will be held accountable."
"France also calls for light to be shed on the alleged use of chemical weapons in the attacks," the ministry said.
In denying the use of chemical weapons, a Syrian government spokesman said the reports were an "attempt to divert the U.N. chemical weapons investigation commission away from carrying out its duties," the state-run Syrian Arab News Agency reported.
Both sides allege use of chemical weapons
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 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 U.N. 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.