Portugal will contribute 100 million euros ($109 million) to a Czech-led initiative seeking to deliver ammunition to Ukraine from third nations, the country's defense ministry said.

The ministry said the initiative aimed to quickly deliver the largest possible quantity of ammunition of various calibers, particularly 155 milimetres.

"The use of these munitions on the battlefield has reached extremely high levels, which makes it vital and urgent for Ukraine to obtain additional munitions to respond to Russia's continued and more intense attacks," it said in a statement late on Thursday.

NATO Secretary General Jens Stoltenberg said on Thursday that Ukraine was running out of ammunition in its war against Russia's invasion and that the allies were not doing enough to help Kyiv.

Ammunition supplies have become a critical issue for Ukraine as fighting against Russia's invasion has entered its third year, descending into grinding artillery battles.