Pentagon chief Jim Mattis said Monday there is “no doubt” the Syrian regime was behind last week´s deadly chemical strike, while US officials accused Moscow of waging a misinformation campaign around the attack.

The charge that Russia is obfuscating who is responsible for the horrific events of April 4 when at least 87 civilians were killed in a rebel-held town came just as US Secretary of State Rex Tillerson touched down in Moscow to confront the Kremlin over its support for Bashar al-Assad.

In televised comments, Russian President Vladimir Putin not only repeated his defence of Assad, but suggested the Syrian leader´s rebel opponents are preparing future faked attacks in order to encourage US intervention.

Putin said Moscow also wanted to see a “thorough” probe of the attack in the town of Khan Sheikhun conducted through the UN and slammed US accusations as reminiscent of the weapons of mass destruction claims leading up to the 2003 Iraq invasion.

A senior US official said Moscow is trying to “confuse the world,” while Mattis said Washington is certain Assad is to blame.

“There is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself,” the new defence chief said in his first Pentagon press conference.

He reiterated warnings that further chemical weapons use will be met with fresh US action: “If they use chemical weapons, they are going to pay a very, very stiff price.”

The United Nations Security Council meanwhile is set to vote Wednesday on a resolution demanding the Syrian government cooperate with an investigation into the attack — a measure Russia will likely veto, diplomats said.