After an opening day of talks marked by clashes between Trump and German Chancellor Angela Merkel, NATO leaders were hoping to focus on policy on Ukraine and Afghanistan.

But Trump set the scene for more tension at the meeting in Brussels, renewing his criticism of European allies -- and Germany in particular.

Trump complained on Twitter that Germany and other NATO allies "pay only a fraction" of the cost of defending Europe, leaving the US to pick up the tab -- a longstanding sore point for the president.

"Germany just started paying Russia, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russia. Not acceptable! All NATO Nations must meet their 2% commitment, and that must ultimately go to 4%!" Trump tweeted.

Apart from the US, only three NATO countries hit the two-percent target in 2017 -- Britain, Greece and Estonia -- but four more are expected to clear the threshold this year.

The gathering is shaping up as the most difficult in years for the alliance that has underpinned Western security for the past 70 years.

Trump, who has said his meeting with Russian President Vladimir Putin next week "may be the easiest" part of his European tour, kicked off the summit with a blistering attack on Germany, calling it a "captive" of Moscow because of its gas links.

- Afghanistan, Ukraine talks -

Trump's demand to double the defence spending target rattled allies, with Bulgarian President Rumen Radev saying that "NATO is not a stock exchange where you can buy security".

NATO Secretary General Jens Stoltenberg said on Wednesday that he would "focus on what we have agreed" on meeting the two-percent target, rather than addressing Trump's four percent demand.

Thursday's talks will focus on Ukraine and Afghanistan, where NATO maintains a training and support mission to help local forces fight the resurgent Taliban.

Leaders have agreed to extend funding for Afghan forces to 2024 and French President Emmanuel Macron said he expected Thursday's talks to proceed "in a much calmer atmosphere than you might think".

Away from the fiery rhetoric, Trump joined the other 28 leaders in backing a joint declaration on Wednesday committing themselves to greater "burden sharing" and to the alliance's founding commitment that an attack on one member is an attack on them all -- with no mention of the four percent.

Croatian President Kolinda Grabar-Kitarovic said talks over dinner had been "very open, very constructive, very positive".

The mood was already prickly ahead of the summit, prompting a terse exhortation from the European Union's President Donald Tusk for Trump to "appreciate" his allies and reminding him that Europe had come to its aid following the 9/11 attacks.

Trump has explicitly linked NATO with a transatlantic trade row by saying the EU shut out US business while expecting America to defend it, singling out Germany for particular criticism.

Europe's biggest economy spends just 1.24 percent of its GDP on defence, compared with 3.5 percent for the US.

- Trump heading to Britain -

Trump has taken particular exception to the proposed Nord Stream II gas pipeline, which is set to run from Russia to Germany under the Baltic Sea.

Merkel, who grew up in communist East Germany, shot back that she knew what it meant to be under Kremlin domination and was glad a united Germany was now able to "make our own independent policies and make our own independent decisions".

European diplomats are wary of a repeat of last month's divisive G7 in Canada, when Trump clashed with his Western allies before meeting Kim Jong Un at a summit, where he praised the North Korean leader as "very talented".

Trump will meet Putin in Helsinki on July 16 for their first summit amid an ongoing US investigation into possible Trump campaign collusion with Russia.

There have been fears that Trump, keen to be seen to make a breakthrough with the Kremlin strongman, might make concessions that would weaken Western unity over issues like Ukraine and Syria.

Trump heads to Britain on Thursday, where the government is in crisis over Brexit and where tensions with Russia have spiked after London blamed Moscow for the death this month of a British woman from contact with the Novichok nerve agent.

The substance is the same military-grade toxin that nearly killed a former Russian spy and his daughter in an earlier attack that Britain also blamed on Moscow.