BUCHAREST, Nov 29 (AFP) - NATO chief Jens Stoltenberg on Tuesday said the alliance would not pull back in its support for Ukraine, calling on partners to pledge more winter aid for Kyiv as it braced itself for more cold and darkness due to Russian attacks on infrastructure.

NATO foreign ministers meeting in Bucharest are focusing on ramping up military assistance for Ukraine such as air defence systems and ammunition, even as diplomats acknowledge supply and capacity issues, but also discuss non-lethal aid as well.

Part of this non-lethal aid - goods such as fuel, medical supplies, winter equipment and drone jammers - has been delivered through a NATO assistance package that allies can contribute to and which Stoltenberg aims to increase.

"Nato will continue to stand for Ukraine as long as it takes. We will not back down," Stoltenberg said in a speech in Bucharest.

He added that the only way to get the right terms for a negotiation to begin would be for Ukraine to advance on the battlefield.

His comments were echoed by several ministers from the 30-member alliance, who were also be joined by Finland and Sweden, as they look to secure full membership pending Turkish and Hungarian ratifications.

"The coming months will be a big test for us all. For Ukraine it is existential, for us moral. We must continue helping Ukraine for as long as necessary," Slovak Foreign Minister Rastislav Kacer said.

Ukrainian President Volodymyr Zelenskiy warned his fellow citizens of new Russian attacks this week that could be as bad as last week's, the worst yet, which left millions of people with no heat, water or power.

Russia acknowledges attacking Ukrainian infrastructure. It denies its intent is to hurt civilians.

"It is going to be a terrible winter for Ukraine, so we are working to strengthen our support for it to be resilient," a senior European diplomat said.

Germany, which holds the G7 presidency, also scheduled a meeting of the Group of Seven rich nations with some partners on the sidelines of the NATO talks as it presses for ways to speed up reconstructing Ukraine's energy infrastructure.

Washington is working with U.S. firms and European nations to locate equipment that can help restore high-voltage transmission stations damaged by Russian missile strikes, a senior State Department official told reporters.

The official did not specify what form the assistance would take or how much it would be worth.

France and Germany are sending more than 100 power generators each to help stabilise the electricity grid.

"The reason Russia is continuing these war crimes is because it is losing ground," a French official said, referring to the attacks on civilian infrastructure.