Trump on Monday announced a tougher line on Moscow over its war in Ukraine as he gave the Kremlin 50 days to end the fighting or face massive new economic sanctions.

Together with NATO chief Mark Rutte he also unveiled a deal under which European members of the military alliance would buy billions of dollars of arms from the United States -- including Patriot anti-missile batteries -- and send them to Kyiv.

Economic powerhouse Germany has already said it is going to play a leading role in the scheme and has offered to buy two Patriot systems.

Denmark's foreign minister Lars Lokke Rasmussen said that his country was "absolutely ready" to join the scheme and would contribute with cash.

"We have to work out the details," he said.

His Dutch counterpart Caspar Veldkamp said the Netherlands was looking into participating in the scheme "with a positive inclination".

"We will look into what we can do in relation to Mr Trump's announcements and take it from there," he said.

One of the major questions hanging over the new plan is how fast it can get desperately needed weaponry to Ukraine as it faces nightly Russian bombardments.

Rutte said that the scheme could allow European countries to send weaponry to Ukraine from their stocks that would be then replaced by new US arms.

But few countries possess systems like Patriots and those that do have frequently argued that they cannot spare more.

He said that beyond Germany, Denmark and the Netherlands, other NATO allies Canada, Norway, Sweden and Britain were among the potential buyers.

The apparent pivot from the US leader away from an attempted rapprochement with Russia's Vladimir Putin will please Washington's European allies.

"After yesterday's announcements, it's clear that leadership is once again coming from the US," said Lithuanian foreign minister Kestutis Budrys.

European countries admit that helping Ukraine keep up its fight against Russia's invasion would be very difficult without access to US weaponry.