US Secretary of State Rex Tillerson has pledged to defend “innocents” around the world ahead of a critical meeting in Russia later this week.

Speaking in Italy before a meeting of G7 Ministers that is expected to lead to an ultimatum for Russian President Putin over his support for Syrian President Assad, Mr Tillerson visited the site of a Nazi massacre at Sant’Anna di Stazzema.

“We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world,” he said.

“This place will serve as an inspiration to us all.”

Monday’s meeting of G7 nations is expected to lead to a directive pledging fresh sanctions against Russia if it fails to cut ties with President Assad.

It follows US air strikes against the Syrian government in retaliation for a chemical weapons attack that killed 72 people including 11 children last week. The deadly attack came after Russia had pledged to rid Syria of chemical weapons and Mr Tillerson was scathing in his criticism of Russia over that failure.

“Either Russia has been complicit or Russia has simply been incompetent in its ability to deliver on its end of that agreement,” he said.

The harsh comments set the scene for a potential showdown between Mr Tillerson and his Russian counterparts. The new Secretary of State will meet with Russian Foreign Minister Sergei Lavrov on Wednesday in the shadow of the Syrian conflict and an FBI investigation over Russia.

The Kremlin has denied a meeting with Russian President Vladimir Putin is on the cards despite the history between the pair.

Mr Putin once awarded Mr Tillerson with the Russian Order of Friendship and the former oil executive faces harsh scrutiny over their dealings.

Over the weekend, Russia warned of a “real war” in tweets asking followers whether they trusted US President Trump and UK Foreign Secretary Boris Johnson to command.

It also asked followers what the “probable outcomes” of the G7 meeting might be, including “conventional war”, a “war of clowns” of a “mix of the above”.

If G7 ultimatum to Russia brings us to real war,what is your trust in @realDonaldTrump as a wartime leader&@BorisJohnson as his lieutenant? — Russian Embassy, UK (@RussianEmbassy) April 9, 2017

If yesterday's statement by @BorisJohnson to be trusted,RTillerson will deliver G7 ultimatum to Moscow next week.What are probable outcomes? — Russian Embassy, UK (@RussianEmbassy) April 9, 2017

The aggressive suggestions come after UK Foreign Secretary Johnson cancelled a scheduled trip to Russia to clear the way for US Secretary of State Rex Tillerson to visit on Wednesday.

Instead, Mr Johnson spent the weekend speaking to his foreign counterparts and drumming up support from G7 ministers for a directive expected to be issued at the G7 meeting in Lucca, Italy on Monday that will likely declare Syrian President Bashar al-Assad must go.

The Russian Embassy said it is “deplorable Boris Johnson found himself unfit to stand Western ground on Syria in bilateral talks with (Russian foreign minister) Sergey Lavrov”.

Russian media also blasted Mr Johnson as a “poodle” and painted a picture of the UK as a US lapdog that is unable to make foreign policy decisions of their own accord.

On Monday, foreign ministers from the G7 — which includes Canada, France, Germany, the US, UK, Italy and Japan — will meet with the aim of pressuring Russia to drop its support for President Assad.

Italian Foreign Minister Angelino Alfano, who is hosting the meeting, said the efforts have brought about a “renewed harmony” between the US and Europe.

The group was formerly known as the G8 and included Russia, but he nation was kicked out after its 2014 annexation of Crimea and support of pro-Russian separatists in the Ukraine.

Ahead of the meeting, Mr Johnson warned US air strikes may not be the last military action we see from the world’s most powerful military towards the Assad government.

“Crucially — they could do so again” he told The Sun about the strikes that followed the sarin gas attack that killed up to 80 people in early April.

“We cannot miss this moment. It is time for Putin to face the truth about the tyrant he is still propping up,” he said.

“I am focused on working with the US and getting the G7 behind the goal of ending this war and rebuilding Syria — one Rex Tillerson will deliver for the whole of the G7 to Russia.”

He added: “There is no doubt that the US action is a game changer in Syria.

“We need to make it clear to Putin that the time to back Assad has gone.”