US President Donald Trump has "taken an important step today", Merz said in a statement from his office.

"President Trump and I have discussed this several times over the past few days. I assured him that Germany will play a decisive role," Merz said.

Trump and NATO Secretary General Mark Rutte unveiled a deal on Monday under which NATO countries will buy arms from the United States -- including Patriot anti-missile batteries -- and then send them to Ukraine.

Rutte said Ukraine would get "massive numbers" of weapons under the deal, which is aimed at placating Trump's long-held complaints that the US is paying too much to aid Ukraine.

"We are doing this in our own interests," Merz said, adding that Germany was still working with its allies to "clarify the details" of the plan.

"Only in this way will pressure on Moscow grow to finally negotiate peace. Ultimately, we are showing that we are acting in unison as security partners," he said.

Speaking during a visit to Washington, Defence Minister Boris Pistorius said Germany had offered to buy two Patriot systems under the deal.

"Germany will continue to play a leading and reliable role in the alliance (NATO)," Pistorius said.