The United States plans to send Patriot missile units to Ukraine to bolster its defenses against Russia's assault on its infrastructure, the White House said.

A White House official, speaking anonymously, said the Patriot air defense system "will be a critical asset to defend the Ukrainian people against Russia’s barbaric attacks on Ukraine’s critical infrastructure."

Ukrainian President Volodymyr Zelenskyy is set to arrive in Washington later on Wednesday when an official announcement will be made.

The move will also send a strong message to Moscow and European allies that Washington is ready to send some of its advanced missile defense weaponry to help Kyiv fight Russian invaders.

What is the Patriot missile system?

Made by US aerospace and defense conglomerate Raytheon, the MIM-104 Patriot is a surface-to-air missile (SAM) system initially developed to intercept high-flying aircraft. It was modified in the 1980s to focus on the new threat of tactical ballistic missiles.

Patriot systems come in fully mobile batteries that include a command center, a radar station to detect incoming threats, and launchers. According to the US-based think tank Center for Strategic and International Studies (CSIS), the current interceptor missile for the Patriot system costs approximately $4 million (€3.8 million) per round and the launchers cost about $10 million (€9.4 million) each.

These US batteries are regularly deployed around the world. In addition, Patriots also are operated or being purchased by the Netherlands, Germany, Japan, Israel, Saudi Arabia, Kuwait, Taiwan, Greece, Spain, South Korea, the United Arab Emirates, Qatar, Romania, Sweden, Poland and Bahrain.

Raytheon says it plans to continue updating the system through at least 2048. Current Patriot batteries can defend against tactical ballistic missiles, cruise missiles, drones, aircraft and "other threats" that the company does not specify.

These are some of the airborne assets Russia uses to attack Ukraine. However, Russian forces also use smaller devices, such as mini drones that keep closer to the ground, which are more difficult for the Patriot system to track and intercept.

The Patriot system covers an area of around 68 kilometers (42 miles), according to the German military. Its radar can track up to 50 targets and engage five of them at once. Depending on the version in use, the interceptor missiles can reach an altitude of more than 24 kilometers and hit targets up to 160 kilometers away.

Each unit requires about 90 troops to operate, according to CSIS.

"We will train Ukrainian forces on how to operate the Patriot missile battery in a third country," the White House official said on Tuesday.

"This will take some time, but Ukrainian troops will take that training back to their country to operate this battery."

Why does Kyiv want the Patriot system?

Ukraine has repeatedly asked Western countries for sophisticated air defense systems to defend against Russia's bombardment of civilian energy infrastructure.

At a recent  Group of Seven (G7) meeting, Zelenskyy specifically called on leaders to send more air defense equipment.

"Unfortunately, Russia still has the advantage in artillery and missiles," he said.

In late November, Ukrainian Foreign Minister Dmytro Kuleba said the Patriot defense system was "what Ukraine needs the most" in order to protect energy infrastructure and stop blackouts.

"We need air defense, IRIS, Hawks, Patriots, and we need transformers," he said from the sidelines of a NATO meeting in Bucharest.

What systems does Ukraine use now?

To battle Russia's low-flying cruise missiles and bomb-like Shahed-136 drones, Ukraine has used a number of different short-range air defense systems, including Russian-made Buks and S-300s, old-generation US-made Hawk missiles, and modern SAM systems like NASAMS.

But modern SAM system launchers and missiles are in extremely short supply. For example, the US can't send any more NASAMS systems until late next year.

The Patriots' biggest value is countering high-flying tactical ballistic missiles. Russia has not used many ballistic missiles in its war on Ukraine, but that could change if it does acquire them from Iran. The Patriots have proven very effective in Saudi Arabia against Iranian-design ballistic missiles fired from Yemen.

What was Moscow's reaction?

Russia has already warned the US not to supply the Patriot air defense system to Ukraine.

Like other heavy weapons, these would become "legitimate priority targets" for Russian forces, Foreign Ministry spokeswoman Maria Zakharova said in Moscow last week. This would once again significantly expand US involvement in the conflict in Ukraine, Zakharova said.

"Washington has already made itself a party to the conflict on the practical level," she added.

Growing amounts of US military assistance, including the transfer of such sophisticated weapons, "would mean even broader involvement of military personnel in the hostilities and could entail possible consequences,'' Zakharova said.

zc,dh/sms (AP, AFP, dpa, Reuters)