Donald Trump's own Defence Secretary has accused Russia of "mucking around in other people's elections" and violating international law.
General James Mattis made the claim following talks with his British counterpart, who accused Russia of “persistent interference” across the globe as tensions continue to rise between the Kremlin and the West.
Sir Michael Fallon said there “cannot be any return to business as usual” with Russia if the behaviour continues.
“There’s a pattern of interference now by Russia in different parts of the globe that needs us to be wary of what Russia is up to,” he told a press conference.
“We need to be extremely watchful of this persistent pattern of Russian interference.”
Sir Michael said Britain would continue to work with Russia to avoid conflict in Syria, where Vladimir Putin’s forces are waging a bombing campaign in support of Bashar al-Assad while the US-led coalition supports anti-Isis opposition forces with air strikes.
Alluding to numerous recent incidents where RAF jets have been scrambled to monitor Russian planes nearing the UK, the Defence Secretary also said his ministry worked to “deconflict” on the edges of British airspace.
General Mattis, who fought in the Gulf War, Afghanistan and Iraq before being picked for the Trump administration, said he was “concerned” about Russian activity in Afghanistan and elsewhere.
“We look to engaging with Russia on a political or diplomatic level but right now, Russia is choosing to be a strategic competitor and we’re finding that we can only have very modest expectations of the areas where we can cooperate,” he said.
“It’s no longer a cooperative engagement with them.”
General Mattis said the US was attempting to diplomatically carve out room for manoeuvre so “Russia can change its behaviour and act in accordance with international norms and international law”.
"Russia's violations of international law are now a matter of record...including mucking around inside other people's elections," he added.
His statements were a stark contrast to those made by Mr Trump, who publicly discredited a report by his own intelligence agencies that found Russia had interfered to turn the US election in his favour.
The President has also drawn criticism for complimentary remarks about Mr Putin made during his election campaign and the transition period.
Investigations continue into Russia’s alleged interference in the US election, which the Kremlin denies, as well as accusations of meddling ahead of the votes in Germany, France and the Netherlands.
Tensions have been rising over Russia’s backing for Assad in Syria, where it is accused of killing civilians with indiscriminate bombing potentially amounting to war crimes, the Ukrainian war and a military build-up in competition with Nato.
Both Sir Michael and General Mattis issued a challenge for members of the military alliance to commit themselves to year-on-year increases in defence spending, calling for “fairer burden sharing” in the military alliance.
It came as reports suggested the Royal Marines may face cuts in response to a £10bn funding shortfall faced by the Ministry of Defence over the coming decade.
Sir Michael declined to rule out a cut in the elite commando units, telling BBC Radio 4's Today programme that while he had not agreed any reduction in the size of the Royal Marines, numbers were always kept “under review”.
But in his conference with General Mattis at Lancaster House, he stressed Britain's military budget was growing every year and remains the largest among European Nato states.
The UK and US are among just five of the alliance's 28 members to meet a target of spending 2 per cent of GDP on defence.
Mr Trump has repeatedly raised the issue, appearing to threaten the withdrawal of US funding after tense talks with Angela Merkel earlier this month.
Responding to previous allegations by Sir Fallon that Russia’s behaviour was becoming more “assertive and arrogant”, the embassy in London dismissed his remarks as a “strong of unfounded accusations”.
“Russia is an independent and sovereign state pursuing its national interest as defined by ourselves,” a spokesperson said, claiming that it was “clearing up the mess” made by Western states in Syria, Libya and Afghanistan.
