U.S. President Joe Biden on Thursday announced he had authorized a new arms package of around $800 million for Ukraine, citing a "critical window" in the conflict as Russia sets the stage for the next phase in the war.

The additional security assistance aims to further shore up support for the embattled European country as it faces a fresh onslaught by Russia on its eastern flank.

"This package includes heavy artillery weapons, dozens of howitzers, and 144,000 rounds of ammunition to go with those howitzers. It also includes more tactical drones," Biden told Americans from the White House's Roosevelt Room.

He called on Congress for supplemental funding to provide additional aid for Kyiv.

Biden stressed the new package was tailored to help Ukraine's forces address the growing Russian offensive in the country's east, which he said would be a different kind of fight than that in the north around Kyiv, where Ukrainian forces successfully beat the Russian invasion back in the first six weeks of the war.

"We’re in a critical window now of time where they’re going to set the stage for the next phase of this war," the president said. The United States and its allies are "moving as fast as possible" to provide Ukraine with the equipment and weapons it needs.

The new arms package is the same size as an $800 million one announced last week but details are still being worked out, a U.S. official, speaking on condition of anonymity, told Reuters earlier.

Biden also announced that Russia-affiliated ships would be banned from U.S. ports and that the U.S. Treasury was putting up a fresh $500 million for Ukraine's government so it could pay salaries and pensions and provide services.

"That means no ship, no ship that sails under the Russian flag or that is owned or operated by a Russian entity will be allowed to dock in a United States port or access our shores. None," the president said.

The U.S. thus joined Canada and European nations, in the latest step to pressure Russia.

On March 1, Canada had shut its ports to Russian-owned ships and barred them from Canadian waters and many European countries have also taken the same step.

The U.S. previously barred Russian airplanes from U.S. airspace, joining Canada and European nations in action, and has banned Russian oil imports, which accounted for much of the prior Russian ship traffic to the U.S.

'Unmistakable message'

Biden said the unity between the U.S. and its allies is sending "an unmistakable message," as he vowed that Russian President Vladimir Putin would never take control of Ukraine.

"Our unity at home with our allies and partners, and our unity with the Ukrainian people, is sending an unmistakable message to Putin – he will never succeed in dominating and occupying all of Ukraine. That will not happen," Biden said.

He pledged that U.S. military aid for Kyiv will not dry up, and said he was preparing to ask Congress for more funds "to keep weapons and ammunition flowing without interruption."

He called on allies to continue their support.

"We have the capacity to do this for a long time. The question is, are we going to maintain the support of the international community to keep the pressure on Putin" and keep enforcing tough sanctions on Russia, he said.

"The most important thing is to maintain unity," he said.

Russia has said it has entered a new stage of its operation and is methodically seeking to "liberate" the Donbass region of eastern Ukraine.

Western allies anticipate Russia's campaign could last many months, grind to a stalemate and test the battlefield capabilities of Ukrainian fighters.

Russia says it launched what it calls a "special military operation" on Feb. 24 to demilitarize and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext.

U.S. forces are not fighting in Ukraine but are indirectly engaged, arming, training and financing its forces.

The U.S. aid announced last week included artillery systems, artillery rounds, armored personnel carriers and unmanned coastal defense boats, broadening the scope of materiel sent to Kyiv to include new types of heavy equipment. read more

Adding this week's package brings total U.S. military aid to Ukraine since Russia invaded to well over $3 billion.