Chairman of the U.S. Joint Chiefs of Staff, General Mark Milley went last week to an undisclosed airfield near the Ukrainian border that has become a hub for shipping weapons, a senior Defense Department official said, seeing firsthand the multinational effort to get weapons into Ukraine amid Russia's invasion, CNN reported.

While at the airfield, Milley met with troops and personnel and examined the shipment activity, the official said. The site has become a beehive of activity in recent days, going from a handful of flights each day to as many as 17, the field's maximum capacity.

The Russian military has not targeted these shipments once they enter Ukraine, the official said, but there is some concern Russia could begin targeting the deliveries as its assault advances.

US European Command (EUCOM) is at the heart of the massive shipment operation, using its liaison network with allies and partners to coordinate "in real time" to send materials into Ukraine, a second Defense official said.

Approximately $240 million of the $350 million US security assistance package has reached Ukraine, and the rest should arrive within days and maybe weeks, "but not longer," the official said.

"I think all of us have been tremendously impressed by how effectively the Ukrainian Armed Forces have been using the equipment that we've provided them," the official said.