"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.
He cited the confidence that the United States said it had a decade ago when it argued that Iraq had weapons of mass destruction.
It's an argument Kerry was prepared for before Congress on Tuesday.
He noted that he and Hagel were senators at the time of the Iraq vote.
"And so we are especially sensitive, Chuck and I, to never again asking any member of Congress to take a vote on faulty intelligence. And that is why our intelligence community has scrubbed and re-scrubbed the evidence."