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 defense 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 defense 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.
- 'A question worth asking' -
Tillerson is the first senior US official to visit Moscow since President Donald Trump took office promising to seek closer anti-terrorist cooperation with Russia, but as he arrived relations were already tense.
Last week, the US fired a volley of cruise missiles at a Syrian airbase in response to the alleged regime attack that left victims foaming at the mouth and gasping for air.
Russia -- which along with Iran has deployed forces to help Assad in the six-year-old civil war -- reacted with fury to the US strike and continues to cast doubt on the regime's involvement in the chemical attack, to Washington's disgust.
US officials have hinted strongly they believe Moscow is at least partially complicit.
"How is it possible that their forces were co-located with the Syrian forces that planned prepared and carried out this chemical weapons attack at the same installation and did not have foreknowledge?" said a senior US official on condition of anonymity.
"We do think it is a question worth asking the Russians."
Still, Tillerson is set to hold talks with his counterpart Foreign Minister Sergei Lavrov on Wednesday, though it's unclear whether an expected meeting with Putin will go ahead.
At the talks, Tillerson will challenge Russia to distance itself from Assad and his Iranian backers and to work with Washington's Western and Arab allies to find a political solution to the conflict with Syria under new leadership.
- Evidence against Assad mounting -
Speaking to reporters before he left a G7 foreign ministers' meeting in Italy for Moscow, Tillerson questioned Russia's alignment 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 flew to Russia, evidence against Damascus appeared to mount.
Assad foe Turkey said blood and urine samples taken from victims of the alleged attack confirmed the banned sarin nerve agent was used.
Russia has remained unmoved by the international chorus of outrage.
The foreign ministry called Tillerson's comments "muscle flexing ahead of the talks" and he is likely to face a chilly reception whether or not he meets Putin.
In Italy, Tillerson attempted to rally the world's leading economies behind fresh sanctions against Assad's regime but, despite much outrage over the Khan Sheikhun killings, he failed to win consensus.
- Regime's 'barbarism' -
But Tillerson and Mattis did go some way towards clarifying the US position on Syria, which had become muddled by apparently conflicting statements from senior officials in Trump's administration about Assad's fate.
Tillerson said the US goal of defeating the Islamic State group in Syria remains a priority, but that Washington hopes a UN-led peace process will allow the Syrian people to oust their own leader.
Ahead of his Moscow trip, Tillerson said at a G7 meeting in Italy that he hoped for a new diplomatic push to end the conflict and create lasting peace for a unified Syria.
"Our hope is Bashar al-Assad will not be part of that future," he said.
In a show of defiance as it came under fire for its ties, Moscow said it would host the foreign ministers of Syria and Iran for three-way talks after Tillerson's visit.
Washington's threat to carry out more punitive strikes if Assad is once again accused of using chemical weapons will likely top their agenda.
Several rounds of UN-backed peace talks have failed to end the Syrian conflict, which has killed more than 320,000 people since March 2011.