The United States on Wednesday announced a new $300 million military aid package for Ukraine that features a slew of ammunition ahead of a planned offensive against invading Russian forces.

The package "includes additional ammunition for US-provided HIMARS (rocket launchers), additional howitzers, artillery and mortar rounds, and anti-armour capabilities that Ukraine is using to push back against Russia's unprovoked war of aggression," the Defense Department said in a statement.

White House Press Secretary Karine Jean-Pierre cited "extensive work by the US government over the past few months to fulfil Ukraine's request ahead of its planned counter-offensive."

The United States has provided more than $35.7 billion in security assistance for Ukraine since Russia invaded the country in late February 2022.

Russian strikes on Ukraine's southern Kherson region killed 21 people and wounded dozens on Wednesday, Kyiv said, as authorities introduced a curfew in the main city of Kherson starting Friday. 

The strikes -- which hit both the city and nearby villages -- came as Ukraine prepares for a spring offensive. President Volodymyr Zelensky said the strikes hit "a railway station and a crossing, a house, a hardware store, a grocery supermarket and a gas station."

Kherson city -- from which Russian forces withdrew last November -- lies near the frontline in southern Ukraine. "As of now, 21 people have been killed! 48 wounded!", Zelensky said on Telegram.

He published images of bodies and wounded people on the floor of a vegetable aisle at a supermarket, with debris around them. "The world needs to see and know this," Zelensky said. 

Kherson prosecutors had called the attack "massive", noting 12 of the victims were killed in the city and others in nearby villages. "On the morning of May 3, Russian troops began the massive shelling of the city of Kherson and the region's settlements," the prosecutors said.

Officials had earlier said three people died in a strike on Kherson's only working hypermarket. The prosecutors said shelling killed three employees of a "power engineering team" between the nearby villages of Stepanivka and Muzykivka.

Officials also announced Wednesday that Kherson will be under curfew from Friday to Monday. Long curfews have been used in  Ukraine in the past to facilitate troop and arms movements.

The head of Kherson's regional military administration Oleksandr Prokudin said the curfew would last from 1700 GMT on Friday until 0300 GMT on Monday. "During these 58 hours, it is forbidden to move on the streets of the city. The city will also be closed for entry and exit," Prokudin said on Telegram, advising residents to stock up on food and medicine.

People could go for short walks near their houses or visit shops but should carry identity documents with them. "Such temporary restrictions are neces