The Ukrainian president Volodymyr Zelensky says that Himars missiles are changing the course of the war against Russia.
The US-made system has been used to hit dozens of Russian targets such as command posts and ammunition depots.
It has also been used to target bridges, including those on the approach to Russian-occupied Kherson, which Ukraine is trying to reclaim.
What is Himars?
Himars - the M142 High Mobility Artillery Rocket System - is a missile launcher mounted on a five-tonne truck which can fire six guided missiles in quick succession.
The missiles supplied to Ukraine have a range of up to 50 miles (80 km), which is over twice the range of the howitzer guns which the US has previously given to Ukraine.
Himars can also fire a single Army Tactical Missile System missile, which has a range of 186 miles (300 km). However, the US isn't supplying Ukraine with these.
The 50-mile range is roughly similar to that of Russian Smerch missiles, but Himars fires GPS-guided missiles which can be more accurately targeted.
How has Ukraine used Himars?
Himars began being used in Ukraine at the end of June or the start of July, according to Phillips O'Brien, professor of strategic studies at St Andrews University.
He says the system was first deployed against fixed targets and command centres, rather than forces on the move:
"It was used up and down the front in eastern Ukraine, striking at ammunition depots some 30 miles behind the Russian lines."
However, he says, Ukraine may now be starting to use Himars against concentrations of Russian troops in barracks and command posts.
Ukraine claims a missile attack in Lysychansk, Luhansk killed up to 100 troops, and that a strike on the headquarters of the Wagner mercenary groupin Popasna in Luhansk may have killed 100 of its members.
Both strikes are likely to have involved Himars, according to Prof O'Brien.
The US is said to have supplied Ukraine with 16 Himars systems as part of its $9bn (£7,47bn) security assistance package.
The UK has also provided a missile-launch system called the M270, which is similar to Himars and fires the same missiles. Three of these have been delivered and three more are on their way.
In mid-July, Ukrainian forces started using Himars to cut off Russia's access to the city of Kherson in southern Ukraine, which it currently occupies.
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Himars missiles damaged bridges across the Dnipro river which link the the city to Russian-annexed Crimea.
"The Russians have been pouring troops into Kherson to try and defend it," says Prof O'Brien.
"The Himars attacks have created a problem in resupplying them."