The United States and Russia were on a collision course over the alleged Syrian chemical attack on Wednesday as Washington’s top diplomat prepared for talks in Moscow.

Secretary of State Rex Tillerson is scheduled for a day of talks with his opposite number Foreign Minister Sergei Lavrov that will now be dominated by the war of words.

But with US officials suggesting Russian forces may have colluded in the latest massacre blamed on Bashar al-Assad’s regime, it is not clear if he would be invited to meet President Vladimir Putin.

On the eve of the meeting, far from trying to calm tempers, both sides escalated their rhetoric.

White House compares Assad tactics to Hitler

Putin accused Assad’s opponents of planning to stage chemical attacks to be blamed on the Assad regime in order to lure the United States, which bombarded a Syrian air base last week, deeper into the conflict.

In a further interview released ahead of the Tillerson talks, the Kremlin leader again slammed the US missile strike and angrily rejected accusations that Assad’s forces were behind the chemical attack.

“Where is the proof that Syrian troops used chemical weapons? There isn’t any. But there was a violation of international law. That is an obvious fact,” Putin told Mir television channel.

For its part, the White House compared Assad’s tactics to those of World War II German dictator Adolf Hitler.

Trump’s spokesman Sean Spicer said Tillerson would go into the meeting with Lavrov to “make sure we let Russia know that they need to live up to the obligations it has made” to halt Assad’s chemical weapons use.

First Moscow visit since Trump presidency

Separately, a senior US official suggested that Russian forces must have had foreknowledge of the chemical attack, which left at least 87 dead in the northern Syrian town of Khan Sheikhun.

And US Defense Secretary James Mattis said Washington has “no doubt” that Assad was behind the massacre.

Mattis said that the US cruise missile strike in response “demonstrates the United States will not passively stand by while Assad ignores international law and employs chemical weapons he declared destroyed”.

Tillerson is the first US cabinet member to visit Moscow since Trump took office after a campaign in which he called for closer ties with Russia, including better cooperation to fight Islamist extremism in Syria.

And the former oil executive might once have looked like the perfect envoy to mend strained ties, having worked closely with Putin’s Kremlin while negotiating deals for energy giant ExxonMobil.