President Donald Trump's defence secretary told Nato allies they must increase defence spending by the end of the year to ensure the US does not "moderate its commitment" to them.

James Mattis also accused some nations of ignoring threats, including from Russia and Isis.

"I owe it to you to give you clarity on the political reality in the United States, and to state the fair demand from my country's people in concrete terms," Mr Mattis said during closed-door talks in Brussels, according to prepared remarks provided to reporters.

"America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to this Alliance, each of your capitals needs to show support for our common defence."

James Mattis says Putin's threat to Nato is biggest threat since WWII

He did not go into detail about how the US might lesson its commitment if Nato members fall short.

Mr Mattis's demands echo those made repeatedly by Mr Trump.

Mr Mattis noted Russia's 2014 annexation of Ukraine's Crimea region and Isis's hold over parts of Iraq and Syria, and said that "some in this alliance have looked away in denial of what is happening."

"Despite the threats from the east and south, we have failed to fill gaps in our Nato response force or to adapt," he added.

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His comments were strongly supported by UK Defence Secretary Sir Michael Fallon, who said all member states needed to meet the Nato target of spending two per cent of GDP on defence.

"There is no uncertainty about America's commitment to Nato. The president reassured our Prime Minister that he is 100 per cent committed to Nato," he said.

"I am looking forward to working with US defence secretary James Mattis on the steps needed to modernise Nato, to make it more agile and more responsive and on how we can persuade other countries to meet their fair share of the burden by increasing their expenditure to 2%."

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Nato officials confirmed that the UK, along with the US, remained one of just five member states to meet the two per cent spending target.

During the election campaign, Mr Trump rattled some Nato members by describing the alliance as "obsolete" and questioning US commitment to the defence of Europe.