US officials announced another $550 million in military aid will be sent to Ukraine, including 75,000 rounds of 155mm artillery ammunition and additional ammunition for High Mobility Artillery Rocket Systems (HIMARS). 

Four more HIMARS, which have a longer range and are more precise than older artillery Soviet-era rocket systems, arrived in Ukraine on Monday morning. 

"We have proven to be smart operators of this weapon," Ukrainian Defense Minister Oleksiy Reznikov tweeted Monday morning. 

Ukrainian forces have used the weapons system to destroy at least 50 Russian ammunition depots since June, Reznikov said previously. 

CHILDREN TRAUMATIZED BY WAR IN UKRAINE FIND MENTORS FROM UNEXPECTED PLACES

In late July, Ukrainians used HIMARS to damage the Antonovsky Bridge, a key crossing that connects Russian forces in the occupied Kherson region to the Crimean peninsula. 

High Mobility Artillery Rocket Systems, or HIMARS

FILE PHOTO: In this May 23, 2011, file photo a launch truck fires the High Mobility Artillery Rocket System (HIMARS) produced by Lockheed Martin during combat training in the high desert of the Yakima Training Center, Wash.  (Tony Overman/The Olympian via AP)

"[The Ukrainians are] spending a lot of time striking targets like ammunition supplies, other logistical supplies, command-and-control," a senior US military official said on a background call with reporters last month. 

"All those things have a direct impact on the ability to conduct operations on the front line.  So I would say yes, although they're not shooting the HIMARS at the front lines, they're having a very, very significant effect on that."

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The Biden administration has sent more than $8 billion in security assistance to Ukraine since Russia invaded, including anti-tank Javelin missiles, anti-aircraft Stinger missiles, drones, and thousands of small arms. 

Reuters contributed to this report.