It is the third package for Ukraine in less than three weeks, following two in late April valued at a total of $7 billion as Washington seeks to make up for months in which it provided only limited assistance to Kyiv.
In a memo released by the White House, President Joe Biden authorized the provision of "up to $400 million in defence articles and services of the Department of Defense, and military education and training" to aid Ukraine.
US Secretary of State Antony Blinken said in a statement that the package contains "urgently needed capabilities" including air defense munitions, artillery rounds, anti-tank weapons, armored vehicles and small arms ammunition.
"The United States and the international coalition we have assembled will continue to stand with Ukraine in its defense of its freedom," Blinken said.
The package was announced on the same day that Russia upped the pressure on Kyiv with a ground offensive into Ukraine's Kharkiv region, which President Volodymyr Zelensky said sparked a "fierce battle."
Officials in Kyiv had for weeks warned Moscow might try to attack its northeastern border regions, pressing its advantage as Ukraine struggles with delays in Western aid and manpower shortages.
The United States has been a key military backer of Ukraine, committing more than $44 billion in weapons, ammunition and other security assistance since Russia launched its full-scale invasion in February 2022.
But prior to April 24, Washington had announced new aid for Ukraine on just one other occasion this year, a $300 million package in March that was only made possible by using money that the Pentagon had saved on other purchases.
Congress had not approved large-scale funding for Kyiv for nearly a year and a half, but took action after months of acrimonious debate among lawmakers.
The US House of Representatives on April 20 approved legislation authorizing $95 billion in aid funding, including $61 billion for Ukraine, while the Senate passed the measure on April 23 and Biden signed it into law the following day.
Washington announced a $1 billion package of military assistance drawn from American stocks shortly after Biden signed the legislation, and followed that up a few days later with $6 billion in security aid that will be procured from the defense industry.