Air defense systems have proved essential to Ukraine as Russia bombards critical infrastructure and civilian targets.

This week, the newly arrived U.S.-made Patriot air defense system shot down missiles aimed at Kyiv, Ukrainian officials said, including advanced missiles that have previously eluded outmatched air defense equipment.

The Patriot is just one of several systems that Western allies have sent to Ukraine since the Russian invasion last year to help shore up Kyiv's defenses. The systems form a grab bag of different capabilities and technologies that are vital in defending the skies from missiles, fighter jets and Iranian-made drones that have terrorized the capital.

But according to leaked U.S. intelligence documents produced in February and March, the supply of air defense systems and their munitions pose a significant and growing problem, as Ukraine uses up their limited stocks to keep up with constant bombardments. Kyiv risks running low on their most common Soviet-era systems, the documents warned, forcing commanders to select what can and cannot be shot down.

That dynamic underscores the inherent challenge, experts have said: Air defense is a difficult mission using expensive and finite resources, with no silver bullets. To address that, the West has ramped up its commitments to Ukraine, sending a range of gear, from truck-mounted guns to the most modern missile-killing systems the United States has to offer.

Here is a rundown of the Western-provided systems Ukraine now has, and what they can do.

Patriot missile system

The Patriot is the U.S. military's most advanced air defense system, with a range of roughly 20 to 100 miles, depending on the threat. It was designed as an antiaircraft system, but newer variants of Patriot can also engage ballistic and cruise missiles and drones.

Along with missiles, a Patriot battery includes radars and control stations to identify, track and target enemy weapons.

The locations of Ukraine's two Patriot systems are a closely kept secret, though one recently used to take down missiles is based in or around Kyiv, according to a U.S. defense official.

Reliance on the Patriot will test the ability of the United States and the West to balance their needs to safeguard their own stocks while providing assistance to Ukraine. Each Patriot interceptor missile costs an estimated $4 million, putting a premium on each decision to fire. Ukrainian officials have said one Russian strategy is to attempt to exhaust air defense systems by saturating the sky with targets, some of which are decoys meant to confuse the interceptor, allowing the real missile to slip through.

NASAM System

The NASAMS suite - short for National Advanced Surface-to-Air Missile Systems - includes a command post, sensors, a radar system and munitions that can be fired from a stand-alone pod or from the back of a truck. This is the air defense system used to protect the White House.

Its radar can detect threats up to roughly 80 miles away, depending on variables including the weather and the size and altitude of a target.

The NASAMS utilizes the same kind of missiles already in common use with Western fighter jets, so their weapon stocks are cheaper and more widely available than the costly Patriot interceptors.

IRIS-T System

The German IRIS-T system is similar to NASAMS. It's a tier below the Patriot system and is configured to use missiles initially designed for fighter jets.

The heat-seeking missile has a medium range of around 20 miles and has proved effective against Russian cruise missiles, said Ian Williams, deputy director of the Missile Defense Project at the Center for Strategic and International Studies. "The problem with [the IRIS-T] is sourcing it," Williams added. Germany has pledged more, but they're still in production, he said.

The Hawk

The Hawk system is the predecessor to the Patriot. Some versions are up to 50 years old.

It is no longer used in the United States, but many U.S. allies still operate the Hawk, including Spain which has sent the system to Ukraine. The United States took some Hawks out of storage to send to Ukraine as well. While the system is older than others Ukraine has, it can be useful against some lower-quality Russian missiles.

Gun trucks

While gun trucks have a much shorter range than other air defense systems, their ammunition is more readily available and cheaper. It's a more basic system that can easily be operated by soldiers who need only to see a threat and fire.

They have been most useful, Williams said, in intercepting Iranian-made drones that Russia has used. "Gun systems have been really good at helping Ukrainians preserve their interceptor capacity," he said. That has helped commanders save more complex and expensive systems for bigger threats, like aircraft and missiles.

The Pentagon said last month it would provide nine such trucks armed with 30mm cannons, along with laser-guided rockets. Both systems, officials said, are intended to be used against drones.

The Washington Post's Karen DeYoung and Serhiy Morgunov contributed to this report.