The United States is still "a ways away" from any possible decision on whether to re-introduce U.S. troops into Ukraine, General Mark Milley, chairman of the Joint Chiefs of Staff, said on Monday, even as he acknowledged low-level planning underway.

President Joe Biden decided to withdraw American troops from Ukraine before Russia's Feb. 24 invasion in order to avoid a direct conflict with a nuclear-armed adversary.

But changing circumstances including a reopening of the U.S. embassy have raised questions about whether U.S. troops may be required to return to help ensure security of diplomats in a country at war.

At a news conference, Milley acknowledged some degree of staff planning ahead of a potential decision to send U.S. troops back into Ukraine. That planning hasn't made it to his level for review or to the level of U.S. Defense Secretary Lloyd Austin.

Then, ultimately, it would be up to Biden.

"At the end of the day, any reintroduction of U.S. forces into Ukraine would require a presidential decision. So we're a ways away from anything like that," Milley said.

"We're still developing courses of action, and none of that's been presented yet to the Secretary."

Milley did not specify whether he was referring to low-level planning for U.S. troops to potentially secure a U.S. diplomatic presence in Ukraine or potentially for other activities as well.

The Pentagon is helping Ukraine battle Russian troops by sharing intelligence and sending it weaponry.

On Monday Austin announced new security assistance packages from 20 countries for Ukraine after hosting a virtual meeting with allies.

That includes a pledge by Denmark to provide Harpoon anti-ship missiles and a launcher to Ukraine, Austin said.

"Everyone here understands the stakes of this war," Austin said.

Tom Karako, a senior fellow at the Center for Strategic and International Studies, said the Harpoon missiles, made by Boeing Co, would be able to help Ukraine threaten high-value Russian ships attacking Ukraine.