President Joe Biden has pledged to "to continue providing Ukraine with the support needed to defend itself," although it is still unclear what that support will look like militarily, Newsweek reported.

Biden told Ukrainian President Volodymyr Zelenskyy that the aid package would include air defense systems. In the past, the U.S. has supplied highly mobile artillery missile systems (HIMARS) and Stingers, and plans to send nationally advanced surface-to-air missile systems (NASAMS).

Gabriela Iveliz Rosa Hernandez, a researcher with the Arms Control Association, told Newsweek that the U.S. government has a policy based on providing Ukraine with weapons that "are relevant for the current fight."  "In theory, Ukraine could use much more advanced systems, such as the Patriot system or the Iron Dome to intercept rockets.The NATO allies could also try to provide more S-300 anti-aircraft systems as they did in the original phases of the conflict because the Ukrainians already know how to use and maintain these systems."

A senior White House official told CNN that the U.S. would continue to help Ukraine with short- and long-range air defense systems, again without going into detail.

In August, Biden announced eight new NASAMS as part of a security assistance package. They have yet to be shipped - two of the eight should be delivered by November.

Hernandez said there are different variables affecting which systems the U.S. decides to provide to Ukraine, adding that providing NASAMS falls under a new phase of U.S. security assistance to Ukraine.

Jordan Cohen, a policy analyst at the Cato Institute, said NASAMS and other systems must be "contracted out" and built before being provided to Ukraine.

"In the arms sales process, most sales usually take years to be delivered, sometimes five or more," Cohen said. "The reason things have been provided so quickly in Ukraine is that much of it has been taken out of existing defense stocks. The two that are expected to be delivered are only two out of the eight. The other six are going to take years to develop, procure and deliver."

Twelve countries currently have NASAMS: the United States, Norway, Finland, Spain, the Netherlands, Oman, Lithuania, Indonesia, Australia, Qatar, Hungary and one unnamed country.

Cohen said the U.S. and NATO are "are in a bind" in deciding what can and should be sent to Ukraine. "The old adage that it is easier to take land than to hold it rings very true right now, especially because of Ukraine's dependence on Western military aid," Cohen said.