Davos, Switzerland - NATO chief Mark Rutte sought to reassure President Donald Trump Wednesday that the alliance would aid the US if it came under attack, after the US leader questioned its commitment to Washington.

"You can be assured, absolutely, if ever [the] US will be under attack, your allies will be with you," Rutte told Trump on the sidelines of the World Economic Forum in Davos, noting that NATO had already done so in Afghanistan following the September 11, 2001 attacks.

Earlier this month, Trump said in a Truth Social post that he doubted allies would "be there for us if we really needed them," writing: "We will always be there for NATO, even if they won't be there for us."

Trump has also repeatedly insisted that the US must take control of Greenland from NATO ally Denmark – a quest that has deeply shaken the global order as well as financial markets.

The US president on Wednesday for the first time ruled out using force to take Greenland, but demanded "immediate negotiations" to acquire the island.

Denmark said it was a "positive" sign that Trump said he wanted to avoid force, but stressed that Trump had not backed away from his push to acquire Greenland.