Russia must abandon its support of President Bashar al-Assad's regime if it wants an "important role" in discussions about Syria's future, US Secretary of State Rex Tillerson said on Tuesday, setting up a clash with Russian leaders just before he met them in Moscow.

President Vladimir Putin's government has chosen to ally itself with an "unreliable partner" in Mr Assad, whose reign is coming to an end, as well as Iran and Hezbollah, Mr Tillerson told reporters in Lucca, Italy, where he was attending a Group of Seven meeting.

"We want to create a future for Syria that is stable and secure," Mr Tillerson said "Russia can be part of that future and play an important role or Russia can maintain its alliance with this group which we believe is not going to serve Russia's interest longer term."

But tough talk on Syria from its closest ally, Russian President Vladimir Putin, made it clear that Mr Tillerson was in for negotiations that would be difficult, if not impossible.

Mr Tillerson's trip, following last week's US missile strikes on Syria, marks the most direct diplomatic effort by the Trump administration to seek an end to Syria's civil war. Russia and Iran back the government of President Bashar Assad, while rebel factions backed by the West and its partners have been largely driven back by withering attacks, including use of a suspected nerve agent on a rebel stronghold that left at least 70 people dead.

In what was in effect an ultimatum, Mr Tillerson said Moscow must calculate the costs of remaining an ally of Assad, the Iranians and Hezbollah.

"Is that a long term alliance that serves Russia's interests?" he told reporters. "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?"

In a sign of escalating tensions, even as Mr Tillerson's plane was arriving in at an airport in Moscow, Mr Putin said in a news conference the the Kremlin has "information" provocateurs are planning to plant chemical substances in suburban Damascus and blame it on Syrian authorities.

Mr Putin said the situation in Syria reminded him of events in Iraq before the US invaded in 2003, an allusion to the nonexistent weapons of mass destruction that the Bush administration used as a justification to invade. He also said Western countries divided over the election of President Trump were scapegoating Russia and Syria.

"We've seen all this before," Mr Putin said at a press conference in the Kremlin with Italian President Sergio Mattarella, describing the chemical attack as "a provocation." 

"Syria and Russia provide a common enemy, a very good platform for consolidation" between the US and its western allies, Mr Putin said. "We're ready to be patient. We hope only that this will end up on some kind of a positive trend."

And the Russian general staff said it has warned the United States not to launch another missile strike in Syria, saying that would be "unacceptable."

'Most difficult period since Cold War'

After days of waffling and insisting Mr Putin had no meeting planned with Mr Tillerson, Russian Foreign Ministry sources told RBC television the two would meet Wednesday.

The ministry laid out its list of expectations for talks that come at a moment when the US-Russian relationship is "in its most difficult period since the Cold War," the ministry said in a statement.

Russia is "concerned about US plans regarding North Korea in the context of a possible scenario of unilateral use of force," the ministry said, mirroring the alarm expressed by senior Russian officials Monday about the possibility of a US strike against Pyongyang.

The Russian side will also expect Washington to agree to "an impartial investigation into the Idlib chemical incident," Moscow's terminology for the chemical weapons attack the United States and its allies have blamed on Mr Assad.

Russia has maintained that a Syrian government airstrike on Idlib hit a factory where Syrian rebels were manufacturing chemical weapons, and Putin's spokesman, Dmitry Peskov, said after the US missile strike that the Syrian government "has no chemical arms stockpiles" and said the strike was based on a "far-fetched notion."

Moscow says that it fulfilled its part of a 2013 agreement mandating that Russia oversee the destruction of Mr Assad's chemical weapons arsenal. On Monday, Russia's general staff said that two locations where chemical weapons might remain are in territory controlled by Syrian rebels.

But Mr Tillerson told reporters last week's poison gas attack shows Moscow did not take its obligations seriously or was incompetent. In either case, he added, the distinction "doesn't much matter to the dead."

"We want to relieve the suffering of the Syrian people," he said, and issued an ultimatum: "Russia can be a part of that future and play an important role. Or Russia can maintain its alliance with this group, which we believe is not going to serve Russia's interests longer term."

Mr Tillerson's visit has the potential to be a window of opportunity, or another marker in the escalation of tensions between between the two great superpowers.

On Monday, the foreign ministry warned that if Washington does nothing to improve relations, "Moscow will react reciprocally."

Russia last week suspended a deal that set up a hotline that allowed Russian and US-led coalition air forces to avoid conflict as they conducted separate operations in the crowded airspace over Syria.

The suspension of that agreement does not mean Russian air defense will shoot down incoming missiles in the event of another US strike, but it will not prevent Syria from defending itself, Viktor Ozerov, the head of the defense and security committee of the upper house of the Russian parliament, told the Interfax news agency.

But Russia would defend itself to ensure the safety of air bases and supply bases in Tartus, he said, wherever a threat originated: by land, air or sea.

Sanctions proposal denied

Mr Tillerson is uniquely qualified to bring a stern warning to the Russians. As the CEO of ExxonMobil, he negotiated a deal with the state-controlled gas company Rosneft, leading Mr Putin to bestow the Order of Friendship on him. Mr Tillerson gained a reputation for being willing to walk out on energy deals that did not meet his standards.

If Mr Tillerson succeeds in nudging Moscow away from Mr Assad, he will have successfully leveraged international outrage over Syria's use of chemical weapons and the US retaliatory strike with the implicit threat it could be used again.

However, the Trump administration still has not explained whether it has a clear strategy to ensure Mr Assad's departure, and what would prompt the United States to take further military action.

And diplomats in Italy did not agree on a British proposal to impose more sanctions on Russia over Syria, on top of sanctions already in place over Ukraine. Italian foreign minister Angelino Alfano said ministers want Russia to pressure Mr Assad, but warned, "We must not push Russia into a corner."

Bloomberg, The Washington Post