Including the supply of a number of Western tanks—one of the Ukrainian defense force’s biggest wish-list items since the war’s start—the latest military aid package will also include the first batch of radar-guided Sea Sparrow anti-air missiles, which can be launched either from land or sea to counter attacks by aircraft or cruise missiles that have become a cornerstone of the Russian offensive.

According to reporting by Politico, the Ukrainian military recently managed to tweak its existing Soviet-era BUK launchers to fire the Sea Sparrow, joining the country with Taiwan as the only nations that have proven capable of operating the ground-launched version of the missiles.

The new supply of missiles also helps to fill a noticeable need for the Ukrainian military as Russia has begun to resort to heavier bombardments to compensate for sizable troop losses on the front lines.

Ukraine’s missile defense has relied on the short- to medium-range National Advanced Surface-to-Air Missile Systems [NASAMS] as well as on Russian-made S-300 missile defense systems as the country’s primary protection against incoming enemy air assaults.

Of the two, the S-300—while an old model—is still considered to be among the most effective defense options in the Ukrainian military’s arsenal, capable of longer ranges and able to “latch onto” incoming targets and blow them out of the sky.

While less versatile than the Sea Sparrow, a favorite of the U.S. Navy, the S-300 is generally considered a superior defense system, able to take out more sophisticated targets with higher rates of success.

However, ammunition for the missile defense system is also prohibitively expensive—and potentially more firepower than the Ukrainian defense actually needs against the comparatively unsophisticated arms Russia has been using in its onslaught against civilian targets across Ukraine.

“[The S-300] is a much more capable system,” Thomas Spoehr, a former lieutenant general in the U.S. Army who now serves as director of Heritage’s Center for National Defense, told Newsweek. “Having said that, it’s a much more expensive system. While the missiles have a longer range, they’re more expensive, and they’re harder to replace.”

Russia has deployed a number of arms against Ukraine in the past, including relatively inexpensive “kamikaze” drones designed for short-range flight.

Meanwhile, the S-300 missiles favored by Ukraine are fairly expensive. An S-300 battery, according to some estimates, can cost anywhere between $120 million to $150 million. A single missile, according to other reports, can cost approximately $1 million.

That’s a cost Spoehr said is difficult to justify—particularly when the objective is countering a deluge of inexpensive armaments sourced from countries like Iran.

“These are not sophisticated drones,” he said. “You don’t need an S-300. And you don’t want an S-300 to counteract those kinds of things.”