CARACAS, Venezuela (AP) — Venezuelan President Nicolas Maduro called on Sunday for an investigation against a leading opposition politician for falsely suggesting the government was carrying out a Syria-like chemical attack against protesters.

Caracas-area Mayor David Smolansky on Saturday denounced the presence of a previously unseen red gas used to scatter thousands of protesters who in recent days have taken to the streets against Maduro.

The type and composition of the gas or smoke is unclear. But some protesters said on social media that its usage amid volleys of tear gas produced nausea and vomiting, while government supporters said it was similar to crowd-control flares used in Spain.

At the height of Saturday's crackdown by police, Smolansky took to Twitter to accuse Maduro of "beginning to use chemical weapons as is occurring in Syria." The tweet immediately went viral and was held up by government critics as a sign of the lengths to which Maduro is relying on security forces to squash dissent.

Maduro responded by calling for an "iron-fisted" investigation against Smolansky, saying his "criminal messages" are an attempt to lay the ground for a U.S. military intervention.

"Venezuela never in its history has had biological, chemical or nuclear weapons," Maduro said on state television.

While heated rhetoric and exaggerated claims are favorite tactics used on both sides of Venezuela's bitter political divide, the singling out of critics in such a forceful way by Maduro is frequently a prelude to legal action.

On Friday, two-time presidential candidate Henrique Capriles was barred from running for public office for 15 years amid an intense campaign waged by the government tying him to the protest movement that has grown into the most combative since a wave of unrest in 2014.