BERLIN -- Chancellor Angela Merkel on Monday underlined Germany's rejection of a claim by U.S. President Donald Trump that her country owes NATO large sums for underspending on defence. She also pointed to Germany's history of decades of post-World War II military restraint.

Trump tweeted Saturday, a day after meeting Merkel in Washington, that "Germany owes vast sums of money to NATO." He added that "the United States must be paid more for the powerful, and very expensive, defence it provides to Germany!"

Berlin's defence budget has long been below NATO's target of 2 per cent of a member's gross domestic product. The figure is currently at 1.23 per cent, though Germany has been raising defence spending and Merkel has stressed its commitment to reaching the target by 2024.

Merkel said defence spending is "not just about contributions to NATO, but also about European contributions in Africa for example, U.N. missions."

"Not a single NATO member state pays its entire defence budget into NATO," she said Monday at a news conference in Hannover with Japanese Prime Minister Shinzo Abe.

Merkel said that defence spending "can't be uncoupled from historical developments from one day to the next." She recalled that the immediate post-World War II aim was to have a Germany that was integrated into the international community.

Germany gradually emerged from its post-war diplomatic and military shell after reunification in 1990, sending troops to Kosovo and Afghanistan -- though it also refused to join the U.S.-led invasion of Iraq in 2003. Military missions aren't popular with the public and are often a subject of agonized political debate.

"Of course the role of Germany has changed," Merkel said.

"If you look at our military commitment today, then you see that a quarter-century later Germany plays a completely different role," she added. "But it is a process, and it is a process that the United States of America wanted ... and we cannot simply cast off this process from one day to the next."

She said that defence spending is only one contribution to security, along with development aid and political solutions to conflicts.

Merkel's centre-left rivals in a September election, and current coalition partners, have struck a sharper tone on the Trump administration's reinforcement of demands that NATO allies pay more.

Spending 2 per cent of GDP on defence would mean doubling Germany's defence budget, and "I don't know who can imagine that something like that is possible," Foreign Minister Sigmar Gabriel told a congress of his Social Democrats on Sunday.