WASHINGTON – The Biden administration will send to Ukraine the most advanced air defense weapon in its arsenal, the Patriot missile system, officials said Wednesday, marking the most significant addition to American military support for the government in Kyiv in months.

The Patriot will be included in a nearly $2 billion package of weapons the Biden administration unveiled Wednesday, marking the visit of Ukrainian President Volodymyr Zelenskyy to Washington.

The visit, during which Zelenskyy visited Biden at the White House and will address a joint session of Congress, comes as Russia carries out a relentless campaign of airstrikes on Ukraine’s civilian infrastructure and electrical grid, which has plunged much of the country into darkness.

Ukraine is to receive one Patriot battery, which has up to eight launchers each that hold between four and 16 missiles, depending on the type of munition used. U.S. forces will train Ukrainians to operate and maintain the system in a third country, probably Germany. “This will take some time,” the official said, “but Ukrainian troops will take that training back to their country to operate this battery.” It is unlikely that the air defense system will arrive in Ukraine before spring.

The new weapons package will bring the total amount of military support for Ukraine approved by the Biden administration to about $22 billion.

Zelenskyy has sought the Patriot system for months, as Russia carries out a barrage of lethal strikes that have extended well beyond military targets, according to U.S. and Ukrainian assessments, and left much of the country without electricity, heat and running water. After months of refusing those entreaties, in recent weeks senior administration officials recommended to Biden that the air defense system be sent to Kyiv.

First used in combat during the Gulf War to take out Iraqi Scud missiles, the Patriot system relies on a sophisticated radar to find incoming threats, including cruise and ballistic missiles, and launches long-range missiles to intercept them. Typically deployed on the back of a truck, it requires a crew of at least three soldiers to operate, with extensive backup needed to keep it functional.

The United States has about 90 troops assigned to a typical Patriot battery. Patriot-launched missiles can fly to altitudes as high as 79,000 feet, with an operational range of up to 100 miles, depending on the ammunition.

A single battery costs about $1 billion, and missiles can run several million dollars each. The systems, produced by Raytheon, are in great demand and short supply, currently in use in about 16 countries. The battery supplied to Ukraine will probably be transferred from systems already deployed in Europe.

The most significant time factor to get the Patriot online in Ukraine is training, which is expected to take months, a senior U.S. defense official told reporters Wednesday, though the Pentagon is eyeing ways to compress some instruction for Ukrainian troops. Experts have said typical training under normal conditions is about half a year.

The Patriot system will join a growing array of other modern air defense systems that Ukraine has received from Western nations since the Russian invasion. The United States last month sent two National Advanced Surface-to-Air Missile Systems, or NASAMS, and signed a $1.2 billion deal to build and provide six more over the next two years.

Earlier in the war, the United States also brokered a deal with Slovakia, a NATO ally, to send its only S-300 air defense system to Ukraine. In exchange, NATO has deployed Patriot units to Slovakia. The Pentagon also has sent surface-to-air missiles known as HAWKs, short for Homing All the Way Killer. The missiles are used for air defense and fired from launchers that Spain provided Ukraine.

Zelensky’s trip is designed to solidify ongoing U.S. support for Ukraine, particularly during the difficult winter ahead. “This week is extremely important for Ukraine – in order to get through this winter and next year,” Zelensky said Tuesday in his nightly address to the country.

“We will definitely endure,” he said. “We will definitely get the necessary support for Ukraine.”

The administration anticipates bipartisan congressional passage of more than $40 billion in additional funding for Ukraine for 2023, the senior official said. Biden “will reinforce the fundamental message on this trip, to President Zelensky, directly to the Ukrainian people, the American people and the world publicly, that the United States will stand with Ukraine for as long as it takes,” the official said.

“This isn’t about sending a message to a particular political party,” the official said. “This is about sending a message to [Russian President Vladimir] Putin and sending a message to the world that America will be there for Ukraine for as long as it takes.”