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 that 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.
Putin accused Syrian rebels 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.
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.
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.
- Underlying tensions -
But the underlying tensions in the relationship never went away and last week's chemical attack, which Moscow denies was launched by Assad's forces, infuriated Trump and led to a punitive US cruise missile strike.
Earlier this week, British Foreign Secretary Boris Johnson cancelled a planned trip to Moscow, but Tillerson maintained his schedule, flying in late Tuesday after a G7 ministerial meeting in Italy.
Speaking to reporters before he left Italy for Moscow, Tillerson said: "Russia has really aligned itself with the Assad regime, the Iranians, and Hezbollah.
"Is that a long-term alliance that serves Russia's interest?" he asked. "Or would Russia prefer to realign with the United States, with other Western countries and Middle East countries who are seeking to resolve the Syrian crisis?"
As Tillerson was flying to Russia, Assad foe Turkey said blood and urine samples taken from victims of the alleged attack confirmed that the banned sarin nerve agent was used.
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.
