Washington: US President Donald Trump asserted on Friday that the NATO allies must pay their fair share for the cost of defense, even as he reaffirmed his "strong support" for the military alliance.

"I reiterated to (German) Chancellor (Angela) Merkel my strong support for NATO, as well as the need for our NATO allies to pay their fair share for the cost of defence," Trump told a White House news conference along with the German leader.

"Many nations owe vast sums of money from past years and it is very unfair to the United States. These nations must pay what they owe," he said soon after the two leaders met at the Oval Office of the White House.

Trump said he thanked Merkel for the German government's commitment to increase defence spending and work toward contributing at least two per cent of GDP.

He also thanked the Chancellor for her leadership in supporting NATO and its efforts in Afghanistan.

"This has come at a significant cost, including the lives of over 50 German soldiers whose sacrifice we greatly honour.

I also appreciate Chancellor Merkel's leadership, along with the French president, to resolve the conflict in Ukraine where we ideally seek a peaceful solution," he said.

The United States and Germany, Trump said, must continue to work together to protect people from radical Islamic terrorism and to defeat ISIS.

"I applaud Chancellor Merkel for Germany's contributions, both civilian and military, as a counter-ISIS coalition member. We also recognise that immigration security is national security," he said.

"We must protect our citizens from those who seek to spread terrorism, extremism and violence inside our borders.

Immigration is a privilege, not a right, and the safety of our citizens must always come first without question," Trump said.

Appreciating Trump's views on NATO, Merkel said its member countries including Germany need to expand their expenditure.

"We committed to this two percent goal until 2024 -- last year we increased our defence spending by eight percent, and we're going to work again and again on this. And we said that, obviously, defence and security has a lot of different assets and facets to it," she said.

She said Germany is going to continue its role in Afghanistan and against ISIS in the Middle East.

"Together, we fight against Islamist terrorism," she said.

"Germany is going to step up its work and is going to continue its work in Afghanistan. Also in Syria, we're going to monitor the situation there very closely. We're going to work on political solutions in Syria but also in Libya -- what we talked about," Merkel said.

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