A letter from Jon Day, chairman of Britain's Joint Intelligence Committee, to British Prime Minister David Cameron, also rejects suggestions that the opposition may have been behind the attack.
"We have tested this assertion using a wide range of intelligence and open sources, and invited (the government) and outside experts to help us establish whether such a thing is possible," Day wrote.
No "credible intelligence" suggests the opposition has chemical weapons, he writes, adding that "there are no plausible alternative scenarios to regime responsibility."
"We also have a limited but growing body of intelligence which supports the judgment that the regime was responsible for the attacks and that they were conducted to help clear the opposition from strategic parts of Damascus. Some of this intelligence is highly sensitive but you have had access to it all."
France gives a similar argument.
"The attack of August 21st could only have been ordered and carried out by the regime," its declassified intelligence report says.
Germany's foreign intelligence agency, the BND, has the same assessment.
"In a secret briefing to select lawmakers on Monday, BND head Gerhard Schindler said that while there is still no incontestable proof, analysis of the evidence at hand has led his intelligence service to believe that Assad's regime is to blame," Der Spiegel reports.
U.N. probe: Limited scope, no clear deadline
The United Nations is pushing all nations to hold off on any action until results of its own examination are in. It's unclear how soon that may be.
"The U.N. mission is uniquely capable of establishing in an impartial and credible manner the facts of any use of chemical weapons," Martin Nesirky, spokesman for Secretary-General Ban Ki-moon, said repeatedly at a news conference Sunday.
But the U.N. probe's mandate is only to determine whether chemical weapons were used -- not by whom.
Obama argued Wednesday that that's no longer in question. "Frankly, nobody is really disputing that chemical weapons were used," he said.
Russia - which, along with China, would likely block any U.N. resolution authorizing military action in Syria -- has repeatedly thrown cold water on suggestions that there's proof of Syria culpability.
"If there are data that the chemical weapons have been used, and used specifically by the regular army, this evidence should be submitted to the U.N. Security Council," Putin said Wednesday in an interview with The Associated Press and Russia's state Channel 1 television.
"It should be a deep and specific probe containing evidence that would be obvious and prove beyond doubt who did it and what means were used."
If there was such evidence, Russia might support a resolution authorizing military strikes, Putin said.
Syria: Allegations 'false and unfounded'
Syria still insists it never used chemical weapons.
"These allegations are false and unfounded," Syria's U.N. Ambassador Bashar al-Ja'afari said Tuesday in an interview with CNN.