Ukraine's president praised the United States for including tank-killing armoured vehicles in its latest multibillion-dollar military aid package, saying they are "exactly what is needed" for Ukrainian troops locked in combat against Russian forces, even as both sides celebrated Orthodox Christmas on Saturday.

The White House announcement Friday (local time) of $3.75 billion ($5.9 billion NZD) in weapons and other aid for Ukraine and its European backers came as Moscow said its troops are observing a short Orthodox Christmas cease-fire.

Ukrainian officials denounced the unilateral 36-hour pause as a ploy and said it appeared to have been ignored by some of Moscow's forces pressing ahead with the nearly 11-month invasion. Ukrainian officials reported Russian shelling attacks in the Dnipropetrovsk and Zaporizhzhia regions on Saturday.

Russia's Defence Ministry insisted Saturday that its forces along the 1,100-kilometre front line were observing the Kremlin-ordered truce, but returned fire when attacked.

The latest package of US military assistance was the biggest to date for Ukraine. For the first time, it included 50 Bradley armored vehicles and 500 of the anti-tank missiles they can fire. Germany also announced it would supply around 40 Marder armored personnel carriers and France promised wheeled AMX-10 RC tank destroyers.

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Together, this week's pledges were powerful signals that Ukraine can count on continued long-term Western aid against Russian President Vladimir Putin's drive to dismember the country.

In his nightly televised address on Friday, Ukrainian President Volodymyr Zelenskyy hailed the US aid package as "very powerful."

"For the first time, we will get Bradley armoured vehicles - this is exactly what is needed. New guns and rounds, including high-precision ones, new rockets, new drones. It is timely and strong," he said.

He thanked US President Joe Biden, US lawmakers and "all the Americans who appreciate freedom, and who know that freedom is worth protecting."

Celebrated by both Ukrainians and Russians, the Orthodox Christmas holiday also underscored the enmity that Russia's invasion is precipitating between them.

In a revered cathedral in Ukraine's capital, Kyiv, the Christmas service Saturday was delivered in the Ukrainian language - instead of Russian - for the first time in decades, highlighting how Ukraine is seeking to jettison Moscow's remaining influences over religious, cultural and economic life in the country.

Ukraine's government on Thursday took over administration of the Kyiv-Pechersk monastery's Dormition Cathedral from the Ukrainian Orthodox Church, which had been loyal to the Russian Orthodox Church, and allowed the Ukrainian church to use it for the Christmas service.

The monastery complex is a UNESCO World Heritage site. The cathedral was built about 1,000 years ago, then reconstructed in the 1990s after being ruined in World War II.

"It's an amazing moment," said Alex Fesiak, among hundreds of worshippers who attended. "Previously this place - on Ukrainian territory, within Kyiv - has been linked to Moscow. Now we feel this is ours, this is Ukrainian. This is part of the Ukrainian nation."

The Putin-ordered Christmas cease-fire that started Friday was first proposed by the Russian Orthodox Church's Kremlin-aligned head, Patriarch Kirill. The Orthodox Church uses the Julian calendar and celebrates Christmas on January 7. Putin's order said a cease-fire would allow worshippers in combat zones to attend Christmas services.

But Ukrainian officials didn't commit to following it and dismissed the move as a Russian ploy to buy time for its struggling invasion forces to regroup. Ukrainian and Western officials portrayed the announcement as a Russian attempt to grab the moral high ground and possibly snatch battlefield initiative and momentum from Ukrainian forces amid their counteroffensive of recent months.

The pause was due to end Saturday night - at midnight Moscow time, which is 11pm in Kyiv.

The Ministry of Defence in Britain, a leading supplier of military aid to Ukraine, said Saturday in its daily readout on the invasion that "fighting has continued at a routine level into the Orthodox Christmas period."