The United States in August approved the transfer of Dutch and Danish US-made warplanes to Ukraine to strengthen Kyiv's Soviet-era air force in its war against Russia.

However, "the planes will only fly in NATO air space" the ministry said in a statement.

"The Romanian training centre will first use the planes for a refresher course for hired instructors. This will be followed by pilot training," it said.

The planes will be maintained by its US-based manufacturer Lockheed Martin, who will also provide the training.

In total, the Netherlands will make 12 to 18 F-16s available for training purposes.

Ukraine's president Volodymyr Zelensky thanked the country for sending the planes and said he hoped they will reach Ukraine "as soon as possible."

"I am grateful to the Netherlands and Prime Minister Mark Rutte for leading the way in supporting Ukraine," he said on social media.

He said the movement of the planes marked a "milestone".

Earlier on Tuesday Zelensky urged Ukraine's allies to provide his military with long range missiles and fighter jets to break an impasse on the front line.