WASHINGTON  - US President Donald Trump unleashed a diatribe against Germany on Saturday, saying Berlin owes Nato "vast sums of money" and must pay the United States more for security. 

His latest tweetstorm comes a day after he met German Chancellor Angela Merkel in Washington, where the two leaders showed little common ground over a host of thorny issues, including Nato and defence spending. 

"Germany owes vast sums of money to Nato & the United States must be paid more for the powerful, and very expensive, defence it provides to Germany!" Trump tweeted on Saturday morning.  

He prefaced his statement by lashing out at the news media. "Despite what you have heard from the FAKE NEWS," he tweeted, "I had a GREAT meeting with German Chancellor Angela Merkel." 

That appeared to be far from the case on Friday, when the veteran German leader arrived hoping to reverse a chill in relations after Trump criticized her during his campaign last year, saying her decision to allow refugees into Germany was a "catastrophic mistake" and suggesting she was "ruining Germany." 

During a joint news conference, Trump accused Germany of unfair trade practices and ripped into Washington's Nato allies, demanding they pay back "vast sums of money from past years." 

Merkel said Germany had committed to increasing its military spending to two percent of GDP, a target Nato member states formally agreed in 2014 to reach within 10 years. 

Trump had made European defence spending an issue during his campaign, saying the United States - which spends just over three percent of its GDP on defence - carries too much of the financial burden for supporting Nato. 

However, at least one critic on Saturday pointed out that Nato members don't pay the United States for security, but contribute by spending on their own militaries. 

"Sorry, Mr President, that's not how Nato works," tweeted Ivo Daalder, a former US ambassador to Nato. "This is not a financial transaction, where Nato countries pay the US to defend them. It is part of our treaty commitment." 

Trump has also worried US allies by criticizing the military alliance as "obsolete" and failing to meet the challenge posed by the terror groups. 

His attacks have come as other Nato members are concerned about Russia's aggressive posture on the continent. Russia annexed the Black Sea peninsula Crimea from Ukraine in 2014 and has backed separatist rebels in eastern Ukraine. 

US defence spending - $679 billion in 2016 - accounts for nearly 70 percent of the total defence budgets of Nato 's 28 members. 

Besides the United States (at 3.36 percent of GDP), only Britain (2.17 percent), Poland (2.01 percent), Estonia (2.18 percent) and Greece (2.36 percent) currently reach the goal, according to NATO estimates for 2016. 

Germany, whose militaristic past has led it traditionally to be reticent on defence matters, currently spends 1.2 percent of GDP. 

But the country's defence minister has called for changes to the way Nato members' commitments to budget targets are assessed. 

Speaking on Friday ahead of Merkel's trip to Washington, Ursula von der Leyen told AFP that the two percent target paints an incomplete picture of actual contributions, saying member states that take part in Nato operations and exercises or contribute personnel and hardware should get credit toward the two percent goal. 

"For me, the question is who is really providing added value to the alliance," she said. 

Von der Leyen proposed using an "activity index" that would take participation in foreign missions into account when assessing budget earmarks for defence.