"I support it," Trump said during a joint news conference with British Prime Minister Keir Starmer on Thursday.
He added: "I don’t think we’re going to have any reason for it. I think we’re going to have a very successful peace, and I think it’s going to be a long-lasting peace, and I think it’s going to happen, hopefully quickly."
Trump's remarks came as he and Starmer held high-level discussions about the future of peace in Europe, US broadcaster CNN reported.
Article 5 of the Washington Treaty guarantees collective defence by stating that an armed attack against one NATO member state "shall be considered an attack against them all."
It has been invoked only once, after the September 11, 2001 terrorist attacks on the United States.
Trump was asked about the clause in the context of a possible European peacekeeping force in Ukraine following a ceasefire, according to CNN.
Trump has previously raised doubts about NATO’s mutual defence commitment, alarming allies, CNN reported.
Last year, he said he would encourage Russia to do "whatever the hell they want" to NATO members that fail to meet defence spending targets.
Referring to the potential ceasefire between Russia and Ukraine, Trump said on Thursday: "If it doesn’t happen quickly, it may not happen at all."
Russia invaded Ukraine by land, air and sea on February 24, 2022, starting the largest military conflict in Europe since World War II.
Thursday is day 1,099 of Russia's war on Ukraine.
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Source: CNN, IAR, PAP