The announcement by Lakhdar Brahimi to reporters in Geneva came as the United States, Britain and France prepared to launch a retaliatory strike against Syrian President Bashar Assad's regime soon, ABC News reported.
"With what has happened on the 21st of August last week, it does seem that some kind of substance was used that killed a lot of people: hundreds, definitely more than a hundred, some people say 300, some people say 600, maybe 1,000, maybe more than 1,000 people," Brahimi said.
Brahimi did not blame either the Assad regime or rebels. He also said international law dictates that "any U.S.-led military action must be taken after" agreement in the U.N. Security Council.
The U.N. chemical weapons inspection team drew its conclusion after spending a day and a half at the sites of last Wednesday's attack.
U.N. Secretary-General Ban Ki-moon, speaking at The Hague, Netherlands, Wednesday, asked that the inspectors be given more time to complete their task and joined Brahimi to urge the international community to work within the U.N. framework.
"The body entrusted with international peace and security cannot be missing in action," he said. "The council must find the unity to act. It must use its authority for peace."
Britain was expected to introduce a resolution to the U.N. Security Council Wednesday that would authorize "necessary measures to protect civilians" in Syria, The Guardian said.
"We've always said we want the U.N. Security Council to live up to its responsibilities on Syria," Cameron posted on his Twitter page. "Today they have an opportunity to do that."
In a speech Tuesday U.S. Vice President Joe Biden said there was "no doubt" the Syrian government had used chemical weapons in an attack last week in the suburbs of Damascus. On Monday, U.S. Secretary of State John Kerry said it was "undeniable" that forces loyal to Assad used chemical weapons in the attack.
In Washington, the White House plans to release evidence as soon as Thursday it said will prove Assad's regime is responsible for the Aug. 21 chemical attack outside Damascus that killed more than 1,000 people, The Washington Post reported Wednesday.
Assad has denied the charges and has accused rebels of using chemical weapons.
Two of Syria's biggest allies -- Russia and China -- are permanent members of the Security Council and have been staunch opponents of anti-Syrian initiatives.
"We are ready to go, like that," Hagel said.
Meanwhile, the Syrian Network for Human Rights said in a report released Tuesday it documented 531 deaths in last week's chemical attack.
"[The] Syrian regime owns almost 700 tons of chemical weapons," the report said. "The quantity is able to manufacture almost 2500 artillery shell and 100 chemical warheads for Scud missiles, which is enough to annihilate the population of Damascus, Homs, Hama and Aleppo."
A Syrian army source told the Iranian news agency FARS that if the Syrian capital was attacked, "Tel Aviv will be targeted too."
"We are rest assured that if Syria is attacked, Israel will also be set on fire," the source said
Iranian Supreme Leader Ayatollah Ali Khamenei told the government backed ISNA news agency that a U.S. military strike, calling it a "disaster" for the region, ABC News reported.
Israeli Army Radio said the country has reinforced its missile defenses as a precaution.
Cameron spoke with President Obama Tuesday night, The Guardian said. While officials said Cameron and Obama had not agreed on the "specific nature" of their response, it was understood they were planning limited missile attacks, likely before the end of the week.
A White House readout of Tuesday's call said the two leaders "discussed possible responses by the international community to the indiscriminate use of chemical weapons on Aug. 21 and agreed to stay in close consultation in the coming days."
Cameron on Tuesday recalled Parliament to allow members to vote on the matter Thursday.
The report on the evidence by the office of National Intelligence Director James Clapper is a final step before Obama decides on a U.S. military strike against Syria, the Post said.
Three days of U.S. missile strikes against Syria could start as early as Thursday, NBC said, citing senior U.S. officials.
Britain, France and Turkey have expressed a willingness to contribute to military action.
CNET.com reported that Syrian Electronic Army, a hacker group that supports Assad, hacked into social media outlets and was involved in the blackout on The New York Time's website Tuesday.