Syria's main opposition group on August 21 accused the government of "massacring" more than 1,300 people in chemical attacks near Damascus.
The Syrian National Coalition said at a press conference in Istanbul that the attacks marked a significant turning point in the civil war. "This time [the attack] was for annihilation rather than terror," coalition member George Sabra said.
Syria's government and military denied that they used chemical weapons in the assaults on rebel-held positions outside Damascus.
White House spokesman Josh Earnest said in a statement that Washington was "deeply concerned" about the report and was formally requesting that the United Nations urgently investigate the allegation.
Meanwhile, the UN Security Council has said it will hold an emergency meeting on August 21 to discuss the latest allegations.
Earnest told a separate media briefing that an investigation should be a "top priority" for the United Nations.
"The United States will be consulting with our allies and our partners on the UN Security Council about this, because this is and should be a top priority of the United Nations," Earnest said.
The alleged attacks came amid a visit to Syria by a 20-member UN chemical-weapons team, led by Swedish expert Ake Sellstrom. The UN team has a mandate to investigate three previous allegations of chemical-weapons use by both sides in the 29-month conflict.
Some commentators have questioned the value of the UN probe, since the team has a mandate to report only on whether chemical weapons were used, but cannot determine the responsibility for an attack.
However, UN deputy spokesman Eduardo del Buey said on August 21 that the UN team that is on the ground in Syria remained "fully engaged" in probing both past accusations of chemical weapons use and the latest alleged chemical attack.
"The United Nations mission to investigate allegations of chemical weapons used in Syria is following the current situation in Syria carefully and remains fully engaged in the investigation process that is mandated by the secretary-general," del Buey said.
Del Buey added that UN Secretary-General Ban Ki-moon was determined to ensure a "thorough investigation" of all reported incidents.
Government, Rebels Trade Accusations
On state television, a Syrian military spokesman rejected claims that the Bashar al-Assad regime used chemical weapons.
"The media channels of sedition and misinformation who shed Syrian blood have lied as usual that the Syrian Arab Army used chemical weapons in the suburbs of Damascus today," the spokesman said.
"The general leadership of the army confirms these allegations are completely false and are a part of the dirty media war that is led by some countries against Syria."
Internet videos purportedly of victims of the chemical attack showed dead bodies on floors with no visible signs of injury.
In Istanbul, Syrian National Council spokesman Khaled Saleh told Reuters that the government forces of Assad were clearly responsible.
"It's very obvious to us that these chemical weapons were used and were carried out using ballistic missiles," Saleh said. "Only the regime has the capability and willingness to use them against innocent civilians."
Arab League chief Nabil Elaraby called on a team of UN chemical weapons inspectors that is already in Syria to "immediately go to eastern Ghouta to see the reality of the situation and investigate the circumstances of this crime."
The European Union condemned the suspected use of chemical weapons by Syrian government forces as "totally unacceptable" and called for an "immediate and thorough" investigation of the alleged chemical attacks.
EU foreign-policy chief Catherine Ashton told journalists in Brussels that charges by the National Coalition "should be immediately and thoroughly investigated."
But Russia's Foreign Ministry said in a statement that the claim could be "a provocation."
British Foreign Secretary William Hague said in a statement that Britain was concerned by reports that "hundreds of people, including children, have been killed in air strikes and a chemical-weapons attack on rebel-held areas near Damascus."
Hague said Britain would raise the claim of the Syrian government's use of chemical weapons at the UN Security Council.
France and Turkey have backed calls to send the UN inspectors to the scene of the attacks, with France pledging to raise the issue at the Security Council.
Syrian authorities and rebels have accused each other of using chemical weapons during the course of Syria's nearly 30-month civil war.