"Delays in delivery of air defences will allow Russian missiles to hit more targets and delays in delivery of ammunition will allow Russia to press along the front line," Stoltenberg said.

"Ukraine simply cannot wait. It needs air defences, ammunition and aid now."

NATO countries last week promised they would look into their inventories after Kyiv begged for more Patriot air defence systems capable of shooting down Russian ballistic missiles.

Moscow has ramped up its aerial bombardments on Ukraine's infrastructure in recent weeks, with President Volodymyr Zelensky saying the Kremlin was aiming to drive people from second city Kharkiv.

Kyiv's desperate appeals for new support come as its outgunned forces are struggling to hold back Russia's military along the front line.

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

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

Washington said Tuesday it has agreed to sell $138 million worth of urgently needed military equipment to Ukraine to repair and upgrade its HAWK missile systems while a larger $60 billion aid package remains stalled.