NEW YORK CITY — President Donald Trump said Tuesday that NATO allies should shoot down Russian aircraft encroaching on their airspace, and walked back earlier support for Russia to keep the Ukrainian territory it has won.

“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,” he wrote on social media shortly after meeting with Ukrainian President Volodymyr Zelenskyy on the sidelines of the United Nations General Assembly.

For months as he tried to coax Russian President Vladimir Putin into peace talks, Trump indicated Ukraine needed to be realistic about its long term prospects and military disadvantage, suggesting it would have to give up the land it had already lost in order to end the war.

Although Trump has been reluctant to increase sanctions on Moscow despite the urging of NATO allies and many Republicans in Washington, his shift in rhetoric — and strategic assessment of the war itself — suggests the president is coming to terms with the fact that his diplomatic overtures to Putin have been unsuccessful.

But Trump’s messages on Ukraine in recent days have been inconsistent at times, ambiguous at others.

Following his February dust-up with Zelenskyy in the Oval Office, Trump has been increasingly conciliatory and at times empathetic in expressing support for Ukraine’s cause, while at the same time growing frustrated with Putin’s refusal to engage in peace talks. But that seemed to change five weeks ago when the Russian leader asked for a summit with Trump in Alaska, where the president emerged from three hours of talks declaring that there was a general agreement in place for direct talks between Putin and Zelenskyy. That, of course, never happened, as Russia has only increased its attacks.

Polish officials appeared pleased with Trump’s declaration of support for shooting down Russian drones. “Roger that,” Polish Foreign Minister Radoslaw Sikorski posted on social media with a link to Trump’s remarks.

Trump hedged Tuesday on whether the U.S. would back up NATO allies in the event of an escalation with Russia, seeming to walk back comments days earlier where he said more clearly that he would.

A potential U.S. response “depends on the circumstance, but we’re very strong toward NATO,” Trump said during the meeting with Zelenskyy.

Trump did not elaborate further and said he was not yet sure about what happened in Denmark, where government officials believe Russian drones may have violated the country’s airspace on Monday night and led to the temporary closure of Kastrup Airport in Copenhagen.

The U.S. president was more emphatic on Sunday when he was asked if the U.S. would back up Poland or the Baltic countries if Russia continued to violate their airspace.

“Yeah, I would,” Trump said then. “I would.”

In the public portion of Trump’s meeting with Zelenskyy, one of several he held following his caustic address to the general assembly, Trump expressed solidarity with Ukraine and praised its military for continuing to fight back against Russian aggression.

Trump again demurred when asked if he continued to trust Russian President Vladimir Putin, who told Trump during a meeting in Alaska last month that he desired to end the war but has since accelerated his bombing campaign against Ukraine’s cities and gone even further in testing NATO’s boundaries with drone and aircraft incursions.

“I’ll let you know in about a month from now,” Trump said.