The videos, posted online Saturday by a US congressional panel, show men and children writhing in pain, apparently foaming at the mouth and vomiting.

Purportedly taken in the aftermath of the August 21 attack outside the Syrian capital, the footage also aired on US broadcaster CNN and has been condemned as too graphic by critics.

But Kerry said it was important American lawmakers know "what is at stake" as they debate whether to authorise punitive strikes on Syrian President Bashar al-Assad's regime.

"The vast majority of members of Congress, House and Senate, are undecided and that's why the videos are being shown and the briefings are taking place," Kerry said during a visit to Paris.

"To make sure everybody understands what is at stake," he said. "Those videos make it clear that this is not something abstract... Those videos make it clear to people that these are real human beings, real children, parents being affected in ways that are unacceptable," he said.

"That is why this is important for people to see."

The 13 videos were shown to members of the Senate Intelligence Committee on Thursday, according to the panel's website.

It said the footage was taken from pro-opposition content posted on YouTube, and carried a warning saying that "viewer discretion is advised."

Kerry is in Europe trying to secure backing from Washington's divided allies for military strikes on Syria.

The US Congress returns from its summer break on Monday to consider President Barack Obama's plans for strikes and UN inspectors are to release a report into the alleged chemical weapons attack by the weekend.

The US accuses the Assad regime of gassing more than 1,400 people to death in the August 21 attack.