The United States will send Ukraine a new military aid package worth $2 billion, a top US official said Thursday, a day before the first anniversary of Russia's invasion.

"Today, the United States announced a further $2 billion in security assistance to Ukraine," National Security Advisor Jake Sullivan said on CNN.

He did not give details of the types of armaments to be expected in the package.

Sullivan, who accompanied President Joe Biden on a surprise trip into Kyiv this week, said US officials were constantly deciding how "to give Ukraine the tools that it needs to win."

He noted that while in Kyiv, Biden had delivered to Ukrainian President Volodymyr Zelensky "an announcement of more artillery, more ammunition, more HIMARS," in addition to previous pledges of US armored vehicles and at a later date, tanks.

HIMARS are a US multiple rocket system that Ukrainian forces have used to devastating effect against the invading Russians.