The United States will deliver a new $300 million military aid package to Kyiv, U.S. National Security Advisor Jake Sullivan said on Tuesday.

"So today, on behalf of President Biden I'm announcing an emergency package of security assistance of $300 million worth of weapons and equipment to address some of Ukraine's pressing needs," he said at a press briefing.

He said that this was possible because of unanticipated cost-savings in contracts that the U.S. Department of Defense negotiated to replace equipment the United States have already sent to Ukraine through previous drawdowns.

"And to put a fine point on it: we're able to use these cost-savings to make this modest amount of new security assistance available right now without impacting U.S. military readiness. And the President has directed his team to use these cost-savings," Sullivan said.

"This emergency package that we're announcing contains a large tranche of artillery rounds and GMLRS for the HIMARS," Sullivan said.

"It is assistance that Ukraine desperately needs to hold the line against Russian attacks and to push back against the continuing Russian onslaught in the east and in other parts of Ukraine. This ammunition will keep Ukraine's guns firing for a period but only a short period," he said.

He said that it is nowhere near enough to meet Ukraine's battlefield needs, and it will not prevent Ukraine from running out of ammunition in the weeks to come.

"It goes without saying, this package does not displace and should not delay the critical need to pass the bipartisan national security bill," he said.