US Defence Secretary Jim Mattis has said the poisoning of a former Russian spy and his daughter in Britain amounts to “attempted murder” by the Russian government.

In remarks to reporters at the Pentagon, Mr Mattis said the attack – which has triggered a worldwide wave of expulsions of Russian diplomats – involved the “pretty obvious” use of a chemical agent.

He said it was the first chemical weapon attack in Europe since World War II.

Asked whether this amounted to an act of war, Mr Mattis said it is part of a pattern of Russian actions that President Vladimir Putin apparently believes can be plausibly denied.

Mr Mattis cited as examples Russia’s 2014 annexation of Crimea and its military intervention in eastern Ukraine, as well as its interference in the 2016 US presidential election.

The retired marine general and former senior Nato commander, said he could recall a time when the US and Russian militaries were training together for international peacekeeping missions amid hope of a post-Cold War partnership.

“That regrettably, by Russia’s choice, is now a thing of the past,” he said.

“Russia has chosen to be a strategic competitor, even to the point of reckless activity,” referring to the poisoning of Sergei Skripal, a former Russian intelligence officer convicted of spying for Britain, and his daughter, Yulia, who are in a critical condition in hospital.

“That’s the only thing it can be called to the innocent people in Salisbury who were exposed, and possibly to the extent of being murdered by this stuff.”

Pressed to be more specific in his accusation, Mr Mattis said: “Attempted murder of a man and his daughter. How’s that for starters?”

“They are doing things they think are deniable,” he said. “So they’re trying to break the unity of the Western alliance,” referring to Nato, which joined in the denunciation of the Salisbury attack, announcing on Tuesday that it will expel seven staffers from the Russian mission in Brussels and will deny the pending accreditation requests of three other workers there.

More than 20 countries on Monday announced they were expelling a total of more than 130 Russian diplomats, including 60 kicked out by the United States, in response to the Salisbury incident.

On Tuesday, Australia said it was kicking out two Russian diplomats whom he described as undeclared intelligence officers. They have been given seven days to leave.

Mr Mattis highlighted Nato’s response, which carried undertones of Cold War-like tension, but declined to say what role he believes Mr Putin had in the Salisbury attack and other actions designed to divide the West.

“I won’t speculate. Certainly he’s responsible as the head of state. We all can draw our own conclusions,” he said.

US President Donald Trump spoke by phone with the leaders of two key Nato allies on Tuesday — French President Emmanuel Macron and German Chancellor Angela Merkel.

The White House said that in both conversations the leaders expressed support for the West’s response to the Salisbury poisoning, including Nato’s expulsion of Russians.