Secretary of State Rex Tillerson flew into Moscow Tuesday to confront the Kremlin over its support for Bashar al-Assad as the US questioned if Russia was complicit in an alleged chemical weapons attack.
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 an alleged regime attack using a suspected nerve agent that killed at least 87 civilians in a rebel-held town, many of them children.
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.
Tillerson did not cancel his visit, however, and he will hold talks with his counterpart Foreign Minister Sergei Lavrov on Wednesday. It is not yet clear whether an expected meeting with President Vladimir 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.
As the top diplomat landed, a senior US official accused Moscow of carrying out a misinformation campaign to "confuse the world" and said Washington was investigating whether Russia was complicit in the alleged chemical weapons attack.
"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 the official, speaking on condition of anonymity.
"We do think it is a question worth asking the Russians."
Speaking to reporters before he left a G7 foreign ministers' meeting in 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 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 that the banned sarin nerve agent was used.
Britain, France and the United States later presented a new UN draft resolution demanding an investigation of the suspected chemical attack in Syria after the Security Council failed last week to agree on a response.
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.
The Russian leader in televised comments on Tuesday not only repeated his defence of Assad but suggested the Syrian leader's rebel opponents were preparing future faked attacks in order to encourage US intervention.
Putin said Moscow also wanted to see a "thorough" probe of the incident conducted through the UN and slammed US accusations as reminiscent of the weapons of mass destruction claims that led to the invasion of Iraq in 2003.
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.
He did go some way towards clarifying the US position on Syria, however, 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 (IS) militant 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.
"To be clear, our military action was a direct response to the Assad regime's barbarism," Tillerson told the G7, insisting that the missile strikes were not the start of a military effort aimed at regime change.
As G7 ministers urged a new diplomatic push to end the conflict and create a lasting peace for a unified Syria, Tillerson said: "Our hope is Bashar al-Assad will not be part of that future."
The ministers however failed to agree on whether fresh sanctions should be imposed on Damascus -- or Russia itself.
"At the moment there is no consensus on new sanctions as an effective instrument," Italy's Foreign Minister Angelino Alfano said, after his British counterpart Boris Johnson had raised the issue.
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.