Lucca, Italy (CNN) US Secretary of State Rex Tillerson has arrived in Moscow for crucial talks -- but without a game plan from key allies about how to deal with Russia's backing for Syrian President Bashar al-Assad.

A meeting of G7 foreign ministers on Tuesday rejected a British plan to impose new sanctions on Syria and Russia in the wake of last week's chemical attack that killed more than 80 people.

Britain had hoped the threat of sanctions would strengthen Tillerson's hand in talks with Russian Foreign Minister Sergey Lavrov.

But the G7foreign ministers chose instead to wait for the outcome of an international investigation into the attack on the rebel-held town of Khan Sheikhoun last week, which prompted the US to launch a barrage of missiles on a Syrian airbase on Friday. Western nations blame the Assad regime for the attack.

Tillerson flew straight from the G7 meeting in Italy to Moscow, where he will become the first member of Trump's cabinet to meet the Russian administration.

But the mood in Russia was bleak. "It is obvious that Russian-American relations are going through the most difficult period since the end of the Cold War," the Russian Foreign Ministry said on Tuesday.

Here's the background to Tillerson's crucial trip to Moscow.

Syria dominates G7 meeting

US Secretary of State Rex Tillerson (center) and British Foreign Secretary Boris Johnson (R) at the G7 meeting in Italy

What happened at the G7? The regular meeting of foreign ministers from the G7 industrialized nations was dominated by the Syria crisis. In the wake of last week's chemcial attack, British Foreign Secretary Boris Johnson spearheaded a plan to impose targeted sanctions against Syrian and Russian military figures. The UK wanted to give Tillerson leverage in presenting an ultimatum to Moscow -- back away from Assad or face further penalties.

Who opposed the plan? Other members of the G7 appear to have been lukewarm on the plan. "There is no consensus on additional new sanctions," the Italian Foreign Minister Angelino Alfano said, adding that sanctions would be counterproductive. Alfano said sanctions risked isolating Moscow: "We must have a dialogue with Russia and we must not push Russia into a corner," he said.

What does the outcome mean? Tillerson, who had no experience of international diplomacy before being appointed Secretary of State, is facing a situation that would test the most seasoned of diplomats, heading to Moscow with only verbal messages of support from key allies. G7 ministers, in the face of conflicting signals on US policy towards Syria and Russia, appear to have chosen to postpone any big decision until after those talks. US President Donald Trump will attend NATO and G7 leaders' summits in late May -- in effect, the diplomatic can has been kicked down the road until then.

Lack of clarity from US

What's the US position? The Trump administration has sent mixed signals on Syria and Russia.

But then came the chemical attack in Khan Sheikhoun, which prompted Trump -- who had hitherto shown no inclination to get involved in international conflicts -- to launch a missile bombardment on a Syrian airbase less than three days later. Russia is Syria's main ally, and used the same airfield as a forward operating base.

Where does the US stand on Syria now? The US position on Syria is still not clear. Trump has made no public statement on Syria since last week's missile attack and there have been no press briefings at the Pentagon. White House press secretary Sean Spicer The US position on Syria is still not clear. Trump has made no public statement on Syria since last week's missile attack and there have been no press briefings at the Pentagon. White House press secretary Sean Spicer said twice Monday that the Trump administration would respond if the Assad regime used barrel bombs against his own people, something that has been a regular occurrence in the six-year civil war. Aides later clarified that this "did not signal a change in Administration policy."

US ambassador to the UN Nikki Haley

What about the role of Assad? US ambassador to the UN Nikki Haley US ambassador to the UN Nikki Haley suggested regime change in Syria was inevitable . But Tillerson appeared more equivocal, saying on Sunday the US priority in Syria was the defeat of ISIS.

Tillerson appeared to harden his stance on Assad after the G7 meeting. Pressed by reporters, he said: "In terms of the future of Bashar al-Assad, it is important to us that we undertake a political process that leads to the final conclusion of how Syria will be governed. It is our policy for a unified Syria that is governed by the people of Syria. I think it is clear to all of us that the reign of the Assad family is coming to an end," he said.

Message for Moscow

What's happening in Moscow this week? Tillerson will hold talks with Foreign Minister Lavrov on Wednesday. Early suggestions that he might meet with Russian President Vladimir Putin were dismissed when the Kremlin confirmed nothing was scheduled.

The omens are not good: before Tillerson's plane was due to land in Moscow, emergency services were called to deal with a dumpster fire. Smoke billowed across the airfield. The strategy, telegraphed by the UK in advance, appears to be to persuade Moscow to drop its backing for Assad in the wake of worldwide revulsion at the chemical attack.

Smoke rises behind a runway just before US Secretary of State Rex Tillerson arrival in Moscow's Vnukovo airport.

What does Russia think? Moscow appears exasperated. When asked about accusations that the Syrian regime was behind last week's chemical attack, Russian President Vladimir Putin responded: "This is very tedious, this story."

What has Tillerson said? Tillerson had strong words for Russia after the G7 meeting. He denounced Moscow's "failure" to uphold commitments it made in 2013 to ensure the elimination of chemical weapons in Syria, and said it had not made enough progress in peace talks to end the six-year civil war.

He also laid down an ultimatum for Russia, saying that Moscow must think about whether to remain an ally with the Assad regime, the Iranians and Hezbollah.

"Is that a long term alliance that serves Russia's interests? Or would Russia prefer to realign with the United States, with other Western countries and Middle East countries that are seeking to resolve the Syrian crisis?"