Borrell said Kyiv was asking its allies to urgently send seven Patriot missile systems to help counter Russian attacks.

"It would be inconceivable that we are not able to provide them given that the Western armies have about 100 batteries of Patriot. And still we are not able to provide the seven they are asking desperately," Borrell said.

NATO countries last week promised Kyiv they would look into their inventories for more air defence systems after Ukraine's foreign minister, Dmytro Kuleba, made a plea for Patriots.

Borrell said he would press European Union foreign and defence ministers again to come up with more air defences for Kyiv, when they meet in Luxembourg later this month.

Kyiv's outgunned forces are struggling to hold back Russia along the front line and Moscow has unleashed a fresh surge of attacks on Ukrainian infrastructure.

More than two years into Russia's war, support from Kyiv's Western allies has faltered.

Europe is struggling to generate enough weaponry and United States aid is blocked by political wrangling in Congress.

"We have to fulfil our moral and political obligation towards Ukraine," Borrell said.

"The situation is extremely difficult. The Russian military machine is running at full speed."

- Defence spending -

Borrell's warning on Ukraine was part of a rallying call from the EU's top diplomat for the 27-nation bloc to step up its efforts on defence in the face of greater threats.

"The possibility of a high-intensity conventional war in Europe is no longer a fantasy," he told a conference in Brussels.

"We have obviously to do everything to avoid it. But in order to deter the adversary, we need to have the means to do so."

The EU has launched a raft of initiatives aimed at reinvigorating its defence industry after years of reduced investment following the end of the Cold War.

Borrell predicted that building its defence would become far more central for the EU and form a "third act" for an organisation created to ensure peaceful trade on the continent.

Since World War II, European countries have relied largely on their alliance with the United States in NATO to ensure their security.

But the spectre of a potential return to the White House for an isolationist Donald Trump after the US presidential election in November has spurred fears Washington may not always have Europe's back.

"The US umbrella that has protected us during the Cold War and after may not be open all the time," Borrell said.

"Maybe depending on who is ruling Washington, we cannot rely on the American support and the American capacity to protect us.

"We have to build our common capacity."