Latvia, a former Soviet republic with a large Russian ethnic minority, received NATO reinforcements on the alliance's eastern flank bordering Russia in the build-up to the war.

The US has deployed 1,400 soldiers in Latvia and dispatched around 20 Apache combat helicopters since Russian President Vladimir Putin launched the invasion of Ukraine on February 24.

General Mark Milley, chairman of the Joint Chiefs of Staff, told local media he US was there to support the Latvian people and army, after reviewing US troops at the Adazi military base near the capital Riga.

Milley said the reinforcements aimed to deter Russian aggression.

Under NATO's Article 5, all members must defend a fellow NATO nation if it is attacked.

But Milley ruled out imposing a no-fly zone above Ukraine -- a measure demanded by Kyiv -- as that would mean NATO would have to enforce it and "actively fight against Russian air forces".

"That is not something that NATO Secretary General (Jens) Stoltenberg or any member state's senior political leadership has indicated that they want to do," said Milley, the first high-ranking US official to publicly speak about the issue.

- 'Second Afghanistan' -

NATO on Friday rejected President Volodymyr Zelensky's demand to establish a no-fly zone over Ukraine to avoid being dragged into a wider European war.

Putin on Saturday warned a no-fly zone would "be considered by us as participation in an armed conflict" and trigger "colossal and catastrophic consequences not only for Europe but also the whole world".

In Riga, Milley met the Latvian army's top officials, who requested more US military aid in the form of aerial defence systems, coastal surveillance ships and artillery, according to Latvian Defence Minister Artis Pabriks.

Riga also wants the US army to establish a permanent presence in the country as opposed to its policy of deploying rotations.

Latvia was one of the first countries to send military aid to Ukraine even before Russia's invasion.

"The Ukrainians are fighting for us. If Ukraine falls, the pressure on Europe, on all the bordering countries (of Russia) will increase drastically," Pabriks told reporters.

He said Ukraine's resistance shows that Moscow cannot control the country in the long term as its forces will face a guerrilla movement akin to "a second Afghanistan for Russia".

"Russia will become a second North Korea," added Pabriks. "We have to do everything to help the Ukrainians hold out."