The aid, which will be detailed by Secretary of State Antony Blinken on the sidelines of a NATO meeting in the Romanian capital Bucharest, "is substantial and it is not the end", one senior official told journalists Monday, speaking on condition of anonymity and without giving further details.

However, he noted that the Biden administration had budgeted $1.1 billion for energy spending in Ukraine and Moldova.

It comes ahead of an international donors' conference on support for the Ukrainian civil resistance to be held December 13 in France, he pointed out.

A Russian campaign of missile strikes targeting Ukraine's energy infrastructure has damaged between 25 and 30 percent of the grid, according to figures cited by Kyiv.

"What the Russians have been doing is targeting these large transformer stations. They are high-voltage transformer stations", not just power plants, the US official said, a move aimed at disrupting the entire energy network from production to distribution.

NATO foreign ministers are meeting Tuesday and Wednesday in Bucharest where the alliance's support for Ukraine since the Russian invasion will be discussed.

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

The United States will call on the other member countries to strengthen their aid in this area, according to the US official.

It will also be an opportunity to highlight the "remarkable cohesion and unity that is still there among all the allies" since the start of the war, assistant secretary of state for European and Eurasian affairs Karen Donfried told reporters on Monday.

Romania, as well as neighboring Moldova, has been hard hit by the war, and around two million people fleeing Ukraine have passed through the country.

Bucharest currently hosts nearly 80,000 refugees, according to figures cited by Washington.

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

They will also discuss the growing threat posed by China. This discussion, initiated at the NATO summit in June in Madrid, "is not about NATO going to China; it is reflecting the challenges really of the PRC having come to Europe", said Donfried, citing the technological challenge in particular.