The United States will provide Ukraine with air defense systems and surface-to-air missiles as part of a new $400 million security assistance package, the Pentagon said on Thursday.

In the face of “Russia’s relentless and brutal airstrikes on Ukrainian civilian and critical infrastructure, additional air defense capabilities are critical,” Pentagon deputy press secretary Sabrina Singh told reporters.

The package includes four highly mobile short-range Avenger air defense systems – the first time they have been delivered to Ukraine – as well as the Stinger missiles that fire them.

Also included in the package are missiles for HAWK air defense systems, which Spain has promised to supply, artillery and mortar shells, ammunition for Himars precision rocket launchers and more than 20 million rounds of small arms ammunition.

The HAWK missiles are being overhauled with funds from Ukraine’s Security Assistance Initiative, which is funding procurement of equipment from the defense industry. Other items in the package will be taken from existing US inventory.

The package amounts to more than $18.6 billion of total US security aid in Ukraine since Russian forces invaded in February.

The security aid announcement came a day after US top general Mark Milley said more than 100,000 Russian military personnel had been killed or wounded in Ukraine, with Kiev’s forces likely to suffer similar casualties.

Milley also said that there was a chance for talks to end the war and that military victory might not be possible for either Russia or Ukraine.

Russia, meanwhile, announced on Thursday that its forces had begun withdrawing from the southern Ukrainian city of Kherson in a major blow to Moscow’s military campaign.

“We are seeing some signs that Russian forces are withdrawing from the city of Kherson, but it is too early to make a full assessment of what that means,” Singh said.