The appeal underscores Ukraine's almost total reliance on its Western allies to down Russian missile barrages, despite having pioneered a system for intercepting long-range drones that is the envy of some of the world's most advanced militaries.

The request comes just days after one of the worst combined missile and drone attacks launched against Kyiv since Russia invaded Ukraine more than four years ago, which wrought devastation across the capital.

In a letter dated May 26 and addressed to President Donald Trump, Zelensky asked Washington to "help us secure this vital tool of protection against Russian terror -- Patriot missiles PAC-3 and additional systems -- to stop Russian ballistic missiles and other Russian missile attacks."

Zelensky conceded in the five-page document, which was also addressed to Congress, that: "when it comes to defending against ballistic missiles, we rely almost exclusively on the United States."

"And it is in Ukrainian hands that Patriot systems have proven something extremely important: The majority of Russian missiles can be stopped," the Ukrainian leader added.

Zelensky's appeal comes at a turbulent moment in ties between Ukraine and the United States.

Trump re-entered the White House last year vowing to bring about a speedy end to Russia's invasion -- now grinding through its fifth year.

- 'Hard to find missiles' -

But US-led efforts to bring Kyiv and Moscow back to the negotiating table have been derailed by the US and Israeli war with Iran, as well as a failure to make progress on key sticking points towards any peace deal, in particular who would control swathes of eastern Ukraine.

Both sides have stepped up their long-range drone and missiles attacks since a series of bilateral talks mediated by the United States earlier this year appeared to stall.

In separate comments to AFP, a senior official within the Ukrainian presidency conceded that finding ammunition for advanced air defence systems supplied by Kyiv's Western allies was "complicated."

"It's just hard to find missiles right now when there are so many other orders in the Gulf and other places like that," the source said.

"And the supplies through PURL have slowed down as well," the source added, referring to a procurement system whereby Ukraine's European allies can purchase weapons from the United States on behalf of Kyiv.

The war in the Middle East, which saw US allies expend huge quantities of air defence ammunition protecting sites in the Gulf, has exacerbated a shortage Ukraine has faced since the start of the war.

Meanwhile, Ukraine's success in drone warfare has attracted the attention of rich Gulf states that have been targeted with the same types of Iranian-designed drones that Ukraine is now well versed in countering.

Russia's invasion of Ukraine was launched by the Kremlin with the intention of speedily toppling the government in Kyiv and forcing the country's capitulation.

More than four years on, it is the bloodiest conflict in Europe since World War II, having cost hundreds of thousands of lives and forced the displacement of millions.