The United States announced a new $300 million arms package for Ukraine on Wednesday, including air defense systems and tens of millions of rounds of ammunition, but warned Kyiv that US weaponry should not be used to attack within Russia.

"We have been very clear with the Ukrainians privately -- we've certainly been clear publicly -- that we do not support attacks inside Russia. We do not enable and we do not encourage attacks inside Russia," National Security Council spokesman John Kirby told reporters.

The Defense Department said the latest shipments will bring the total value of US security assistance to Ukraine to $37.6 billion since Russia's February 2022 launch of the invasion.

"The United States will continue to work with its allies and partners to provide Ukraine with capabilities to meet its immediate battlefield needs and longer-term security assistance requirements," the Pentagon said in a statement.

The United States has been leading an unprecedented effort by NATO and other allied countries to supply Ukraine with weaponry and other aid.

The latest arms shipments come as Ukraine prepares a counter-offensive aimed at driving Russian forces back from swaths of occupied territory in the east and south of the country.

The assistance also comes after a spate of attacks made in shadowy circumstances on targets inside Russia itself, including an unprecedented barrage of drone attacks on Moscow.

Kirby said the United States has laid out its ground rules to Ukraine.

"We don't tell them where to strike. We don't tell them where not to strike," he said. "Ultimately, President (Volodymyr) Zelensky and his military commanders decide what they're going to do."

However, "we certainly don't want to see attacks inside Russia that are being propagated, that are being conducted, using US-supplied equipment."

Kirby said that despite the growing tensions over the issue, the White House remains confident that Ukraine will keep its promise not to use US-built F-16 warplanes -- set to be supplied by European countries -- against targets beyond Ukrainian borders.

"We have gotten that assurance at various levels," Kirby said.

Kirby said that while Washington is fullheartedly backing the Ukrainian effort to defeat the Russian invasion, it wants to avoid situations that "suck in the West and NATO and the United States" and to "avoid World War III."

The Pentagon said the $300 million package includes munitions for Patriot air defense systems, AIM-7 air defense missiles, Avenger air defense systems and Stinger anti-aircraft missiles.

Also part of the package is ammunition for High Mobility Artillery Rocket Systems (HIMARS), 155mm and 105mm artillery rounds, 105mm tank ammunition and Zuni aircraft rockets.

The United States is also sending more than 30 million rounds of small arms ammunition to Ukraine, the Pentagon said.