US President Donald Trump said he would back NATO countries that choose to shoot down intruding Russian planes, but said direct American involvement would depend on the circumstances.

Donald Trump, in a dramatic shift from his call to end the war, also said he believed Ukraine could win back all territory lost to Russia. He posted on social media soon after meeting with Ukrainian President Volodymyr Zelensky on the sidelines of the United Nations General Assembly gathering of world leaders.

“I think Ukraine, with the support of the European Union, is in a position to fight and WIN all of Ukraine back in its original form,” Trump wrote on Truth Social.

“With time, patience, and the financial support of Europe and, in particular, NATO, the original Borders from where this War started, is very much an option,” he added.

When asked by a journalist if he thinks NATO countries should shoot down Russian aircraft if they enter their airspace, Trump replied firmly, “Yes, I do.”

When asked if the US would back up its NATO allies in such a situation, Trump said it “depends on the circumstance” and praised the military alliance for agreeing to ramp up defence spending.

“We are very strong toward NATO. NATO has stepped up. You know, when they went from 2% to 5% that was great unity,” Trump said about new military spending targets.

Donald Trump on Vladimir Putin

On being questioned about his current trust in Russian President Vladimir Putin, Trump said, “I will let you know in about a month.”

Trump’s comments mark the latest in a series of reversals on Ukraine, including a sudden pivot to peace talks with Vladimir Putin early this year that stunned many of his allies.

The US leader had also shown lukewarm support for Ukraine, telling Zelensky during a televised Oval Office bust-up in February that “you don’t have the cards” to beat Russia.

But his new shift reflected his growing frustration with Putin since a summit in Alaska on August 15 failed to produce a breakthrough, and was instead followed by increased Russian attacks.

On Tuesday, Trump told reporters that he had counted on a bond with Putin to bring an end to the three-and-a-half-year-old war, but “unfortunately, that relationship didn’t mean anything”.

In his post, Trump said he had changed his view on the war “after getting to know and fully understand the Ukraine/Russia Military and Economic situation and, after seeing the Economic trouble it is causing Russia.”

“Putin and Russia are in BIG Economic trouble, and this is the time for Ukraine to act,” Trump said.

He added that Russia was “fighting aimlessly” after more than three years of war and was now a “paper tiger” instead of a serious nuclear force.