WASHINGTON -- The timeline has changed for a decision on punishing Syria for a suspected chemical weapons attack. The president tweeted on Thursday that an attack "could be very soon or not so soon at all."

Defense Secretary James Mattis left the White House late Thursday afternoon after briefing the president on options for a military strike against Syria. But shortly afterward the White House said "no final decision has been made." That was a marked change from Monday when a decision seemed imminent.

U.S. intelligence estimates 42 people died in the weekend attack, most of them showing symptoms consistent with chlorine gas, with others having symptoms more consistent with exposure to a nerve agent.

The president of France said flatly that "we have proof that last week chemical weapons, at least chlorine, were used by the regime of Bashar al-Assad." A spokesman for Britain's prime minister said her cabinet agreed it was "highly likely" the regime was responsible.

But testifying before Congress Thursday morning, Defense Secretary James Mattis seemed more cautious.

"I believe there was a chemical attack and we're looking for the actual evidence," Mattis said.

U.S. officials say blood samples provided by medical workers confirm the presence of chemicals but do not tell which side used them -- the regime or, as the Russians have claimed, rebels seeking to overthrow the regime. An international team of experts is expected to arrive in Syria on Saturday, but even if they are allowed to visit the scene of the attack, they will not be able to determine who did it.

"They can only say they found evidence or did not, and as each day goes by, as you know it's a non-persistent gas, so it becomes more and more and more difficult to confirm it," Mattis said.

Mattis is also concerned a strike against Syria could set off a confrontation with Russia. He has cancelled plans to be out of town and President Trump is speaking with the leaders of both France and Great Britain.