ADAZI, LATVIA (NYTIMES) - The United States and the other 29 countries that make up the North Atlantic Treaty Organisation (Nato) remain unwilling to establish a no-fly zone over Ukraine, said President Joe Biden's senior military adviser on Saturday (March 5).

This is even as Ukrainian President Volodymyr Zelensky continues to push the alliance to more forcefully protect his country from the carnage wrought by repeated Russian air strikes in recent days.

"If a no-fly zone was declared, that means someone would have to enforce it," said the adviser, General Mark Milley, chair of the Joint Chiefs of Staff. "That means someone would have to go in and actively fight against Russian forces."

Gen Milley was in Latvia visiting US troops deployed to the heart of Nato's so-called eastern flank when he laid out in stark terms to reporters why the alliance had rejected Mr Zelensky's calls for Nato to set up and enforce a no-fly zone.

Actively fighting Russian forces was "not something" that "any member state's political leadership has indicated they want to do", said Gen Milley.

Mr Zelensky, under bombardment from a Russian military that has repeatedly shelled civilian targets and that far outguns his own forces, has pleaded for the West to declare the skies above Ukraine off-limits to Russian warplanes.

If enforced, such a declaration would at least protect hospitals, schools, homes and other civilian targets from the barrage of air-launched missiles that Russian planes have directed at Ukraine, advocates say.

More than a week after Russia began its invasion of Ukraine, Mr Biden - along with other Nato countries - continues to refuse to send troops into Ukraine to help, lest they be drawn into a hot war with Russia. That applies to the skies as well, Nato officials say.

Russian President Vladimir Putin has said any nation that imposes a no-fly zone would be considered an enemy combatant.

The US and Nato have rushed weapons and ammunition to Ukraine as its military battles Russian forces.

After leaders of the alliance met last Friday in Brussels, Nato secretary-general Jens Stoltenberg said its members had rejected any possibility of physically intervening against Russian forces.

Gen Milley also spoke on recent comments by Mr Putin threatening a nuclear option, saying that "any time any national leader talks about nuclear weapons, obviously that's something that all of the countries of the world take very, very seriously".

He said the US had not yet seen a change in the posture of Russia's nuclear arsenal. But, he added, "that doesn't necessarily mean that something in the future couldn't happen".

Latvian Defence Minister Artis Pabriks was quick to praise the arrival of US troops in his tiny country - just part of Mr Biden's recent deployments to reassure Eastern European allies that the US will use its military might to support Nato countries that border Russia, should Mr Putin seek to venture beyond Ukraine.

"I was woken shortly before 4 o'clock on 24th of February when the invasion started," Mr Pabriks said. "And about four hours later, your battalion was landing in Latvia."

He was referring to the arrival of US paratroopers from the 173rd Airborne Brigade, who landed in Latvia from Vicenza, Italy, on hours' notice. Their arrival, he said, was "seized by the public" in Latvia because "Americans are standing with us".

Latvia, which has fewer than two million residents, has armed forces of around 17,000 troops. But officials were talking tough on Saturday, their anger over Russia's invasion of Ukraine palpable.

"He can't cross our border," Mr Pabriks said, and then referenced Russian airborne forces that have threatened Latvia in the past.

"We will shoot them," he said. "They don't have the forces here to go in, because that airborne brigade - we have been major targeted by that brigade. That brigade is now dying at Kyiv."

Follow The Straits Times' live coverage on the Ukraine crisis here.