Obama pledges to defend Poland's security

“Poland will never stand alone. But not just Poland - Estonia will never stand alone. Latvia will never stand alone. Lithuania will never stand alone. Romania will never stand alone,” he added.

“These are not just words. They’re unbreakable commitments backed by the strongest alliance in the world and the armed forces of the United States of America.”

He did not name Russia as the potential aggressor.

He noted that Nato's recent decision to send extra units to eastern Europe is “not [meant] to threaten any nation” and he spoke of “growing trade” with Russia if relations improve.

But he spoke amid concern that Russia could stir trouble beyond Ukraine to test Nato solidarity.

Obama criticised Russia’s "dark tactics" and its "aggression in Ukraine”.

“Bigger nations must not be allowed to bully the small, or impose their will at the barrel of a gun or with masked men taking over buildings. And the stroke of a pen can never legitimise the theft of a neighbour’s land,” he said, referring to its attack on east Ukraine and its annexation of Crimea.

He spoke on the 25th anniversary of Poland’s first post-Communist election and compared the Polish Cold War struggle to Ukraine’s pro-Western revolution.

“The Ukrainians of today are the heirs of Solidarity - men and women like you who dared to challenge a bankrupt regime,” he said.

Obama on Wednesday morning also met with Ukraine’s new leader, Petro Poroshenko.

He told press afterward: "It’s clear that he understands the aspirations and the hopes of the Ukrainian people ... and I have been deeply impressed by his vision, in part because of his experience as a businessman, in understanding what’s required to help Ukraine grow."

He noted the US is helping to "train and professionalise" the Ukrainian army.

The White House the same day published details of US military aid so far.

It has earmarked $23 million for non-lethal assistance since the outbreak of the Russia-Ukraine conflict. The latest tranche is being spent on body armour, night vision goggles, and communications equipment. Previous money went on: meal kits for Ukrainian soldiers; medical supplies; helmets; barbed wire; bomb-disposal equipment; binoculars; flashlights; and walkie-talkies.

But speaking to the Reuters news agency in Brussels on Wednesday, Nato military chief Philip Breedlove said Ukraine wants more.

"They have asked for everything from command and control, communications, help with cyber, help with kinetic capabilities [weapons], help with training, to developing their national guard forces," he said.

"That [sending weapons] is being discussed and it is inappropriate to remark [on it] at this time".

Crowd pleaser

Obama’s speech pleased the crowd in Warsaw’s Old City, which interrupted with applause on several occasions.

It was also welcomed by the Polish establishment. “Moving and powerful speech,” Polish foreign minister Radek Sikroski tweeted.

But Sikorski, in an interview with the New York Times on Tuesday, also asked the US for more.

Obama has promised to send extra troops to eastern Europe, but on a “rotational” basis until the Russia crisis ends, while Poland wants permanent new Nato facilities.

“There are major [Nato] bases in Britain, in Spain, in Portugal, in Greece, in Italy … Why not here?” Sikorski told the US daily.

“Russia is testing the strength of the international system set up by the United States after World War II. She tested it in Georgia [in the 2008 war], which was an implied ally of the United States. She has now tested it in Ukraine. And I don’t think we can discount the possibility that she will test it again.”