The new weapon is the Himars multiple launch rocket system, or MLRS: a mobile unit that can simultaneously launch multiple precision-guided missiles.

Both Ukraine and Russia already operate MLRS, but the Himars has superior range and precision.

President Joe Biden wrote in The New York Times that the advanced rockets will enable the Ukrainians "to more precisely strike key targets on the battlefield in Ukraine."

Yet the US plans to limit the range of the missiles it gives Ukraine to avoid them being used to hit targets deep inside Russia.

"We are not going to send to Ukraine rocket systems that can strike into Russia," Biden said Monday.

What system will US provide?

The M142 Himars system (High Mobility Artillery Rocket System) is a modernized, lighter and more agile wheel-mounted version of the track-mounted M270 MLRS developed in the 1970s for US and allied forces.

The Himars Washington is providing to Ukraine will have a range of about 50 miles (80 kilometers), a US official told reporters.

Himars carry one preloaded pod of six 227 mm guided missiles (the M270 carries two pods), or one large pod loaded with an ATACMS tactical missile.

With a small crew, the Himars can remove a spent pod and load a fresh one in minutes, without other vehicles helping. The crews will require some training.

The US military already has Himars units in Europe, and NATO allies Poland and Romania have also acquired the systems.

It was not clear how many of the systems the US will send to Ukraine.

Why are they valuable?

The US MLRS will give Ukraine's forces the ability to strike further behind Russian lines, and from distances better protected from Russia's own long-range weaponry.

The GPS-guided missiles the Himars shoots from its six-pack pod have a range about double that of the M777 howitzers that the US recently supplied to Ukraine forces.

That roughly 80 kilometers generally puts the Himars out of the range of Russia's own artillery, while placing the Russian batteries at risk.

It also could threaten Russian supply depots, amid western belief that the Russian forces suffer logistical problems.

The US will not supply Ukraine with the ATACMS, which has a range of 300 kilometers.

Some analysts say Himars would be a "game-changer" in the war at a time when Ukraine forces appear to be struggling under Russian artillery fire.

But others say Himars will not suddenly turn the tables in the three-month-old war.

"The Himars would even the playing field," a senior US defense official said.

Why is Washington limiting the range?

Since Russia invaded on February 24, the United States has been sensitive to taking any action to support Kyiv that might provoke Moscow to take the war beyond Ukraine's borders.

That has included not overtly backing Ukraine strikes inside Russian territory. Several times Ukraine has used its own rockets, drones and helicopters to hit short-range Russian targets in neighboring Kursk and Belgorod oblasts.

If the US provided the ATACMS for use on the Himars by Ukraine, they would theoretically have the ability to strike major Russian urban centers and military bases, including airfields from where attacks on Ukraine are launched.

The "Ukrainians have given assurances they will not use these systems against Russian territory," a US official said.