President Donald Trump fired back a fiercely worded response to London Mayor Sadiq Khan on Monday morning, in an escalating war of words at the start of the president's official state visit to the U.K.

Building on comments to reporters Sunday night, Trump took to Twitter just moments before touching down at Stansted Airport outside London. "He is a stone cold loser who should focus on crime in London, not me," Trump said of the mayor.

He said Khan has "by all accounts" done a terrible job and has been "foolishly 'nasty' to the visiting President of the United States, by far the most important ally of the United Kingdom."

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In an opinion article published ahead of the U.S. president's visit, Khan likened Trump to the fascists who roiled Europe in the 20th century.

"In years to come, I suspect this state visit will be one we look back on with profound regret and acknowledge that we were on the wrong side of history," Khan wrote in The Guardian.

Trump and Khan have had a contentious relationship for years. After attackers killed eight people near London Bridge in 2017, Trump criticized Khan's handling of the attack, and Khan's office called Trump "ill-informed." Khan then urged the U.K. government to reassess its invitation to Trump for a U.K. visit later that year. "I don't think we should roll out the red carpet to the president of the U.S.A. in the circumstances where his policies go against everything we stand for," Khan told U.K. broadcaster Channel 4.