NATO Secretary-General Jens Stoltenberg on Tuesday urged leaders of the alliance heading to a summit in the Spanish capital Madrid to keep up their backing for Ukraine as it faces an onslaught from Russia.

"It is extremely important that we are ready to continue to provide support because Ukraine now faces a brutality which we haven't seen in Europe since the Second World War," NATO chief said ahead of the gathering in Spain.

NATO allies have funneled billions of dollars of arms to Kyiv – including increasingly heavy and longer-range weapons – as it battles to hold back the Kremlin's forces.

The alliance is set to agree on an additional joint package at the summit in Madrid that will include secure communications equipment, anti-drone systems and training to help Ukraine switch to more modern Western weapons in the longer term.

Ukraine's President Volodymyr Zelenskyy, who is set to address NATO leaders, tweeted that he had spoken to Stoltenberg in the run-up to stress "the importance of a powerful missile defense system for Ukraine to prevent Russian terrorist attacks."

That call came after a Russian missile strike on a mall in the central Ukrainian city of Kremenchuk, which drew angry condemnation from Kyiv's Western backers.

Russia's invasion of Ukraine has shaken NATO and reshaped the security situation in Europe.

Stoltenberg has said the U.S.-led military alliance will unveil the largest overhaul of its defense and deterrence since the end of the Cold War.