Moscow has unleashed waves of strikes against Ukraine's energy infrastructure as its troops are pushed back on the ground, plunging millions of people into darkness.

Ukraine's Foreign Minister Dmytro Kuleba called for supplies of weapons, especially advanced air defence systems, to come "faster, faster, faster" as he joined a two-day meeting of NATO foreign ministers in the Romanian capital Bucharest.

"When we have transformers and generators, we can restore our system, our energy grid, and provide people with decent living conditions," Kuleba said.

"When we have air defence systems, we will be able to protect this infrastructure from the next Russian missile strikes."

"In a nutshell, Patriots and transformers is what Ukraine needs the most", he said, referring to the US-made missile defence systems.

The appeal came after NATO chief Jens Stoltenberg accused Russian President Vladimir Putin of targeting infrastructure in a bid to use the winter as "a weapon of war" against Ukraine.

Stoltenberg said the message from NATO allies was "we need to do more" to help Kyiv fix its gas and electricity infrastructure and provide air defence to help it protect itself better.

He said he expected Russia to carry out more attacks on Ukraine's grid as the Kremlin suffers defeats on the ground and warned Europe should "be prepared for more refugees".

"Russia is actually failing on the battlefield. In response to that they are now attacking civilian targets, cities, because they're not able to win territory," Stoltenberg said at the start of the meeting.

US Secretary of State Antony Blinken has announced a package worth $53 million "to support acquisition of critical electricity grid equipment" by Kyiv.

A senior US official said the assistance would "not be the end" and pointed out the Biden administration had budgeted $1.1 billion for energy spending in Ukraine and neighbouring Moldova.

- 'Keep calm, give tanks' -

"This targeting of civilian infrastructure, of energy infrastructure is obviously designed to try and freeze the Ukrainians into submission," said British Foreign Secretary James Cleverly.

"I don't think it'll be successful."

Allies have given arms worth billions of dollars to Ukraine, but Kyiv is pleading for more air defence, tanks and longer-range missiles to push the Kremlin's forces back.

But there are growing concerns that weapon stores in some NATO countries are running low as stockpiles have been diverted to Ukraine.

Lithuanian Foreign Minister Gabrielius Landsbergis said his request to fellow NATO ministers was simple: "Keep calm and give tanks".

Germany, which currently chairs the G7, convened a meeting Tuesday afternoon on the sidelines of the NATO gathering to discuss the energy crisis caused by the war in Ukraine.

- 'Door is open' -

NATO says the meeting in Bucharest has showcased its unity on continuing to support Ukraine as Moscow's war against its neighbour drags into its tenth month.

The alliance will not, however, make any progress on Ukraine's request to join, initially made some 14 years ago when NATO first pledged that Kyiv would one day become a member.

Stoltenberg insisted that the "door is open" to new members but said the focus now was on assisting Ukraine in its fight with Moscow.

NATO has bolstered its eastern flank in the face of Russia's war by sending more troops and equipment to countries neighbouring Ukraine, like Romania.

Non-NATO Moldova, which has also seen blackouts caused by the fallout from the attacks on neighbouring Ukraine, will attend the alliance's talks on Thursday along with Bosnia and Georgia.

Besides the war in Ukraine, the ministers will take stock of progress in the accession of NATO candidates Finland and Sweden, already ratified by 28 of the 30 member countries but which remains suspended awaiting the green light from Hungary and Turkey.

The Finnish, Swedish and Turkish foreign ministers met on the sidelines of the meeting, but Ankara did not signal that there had been any steps forward.