Howitzers are long-range weapons favoured by the US Army and Marine Corps, and have been deployed in numerous global conflicts. The M777 howitzer is one type of two 155mm howitzers, which can hit targets up to 18 miles away.
It is typically operated by around 10 people, forming part of a bigger group of six guns called a battery.
The US 7th Army Training Command is instructing between 50 and 60 Ukrainian officers on the specifics of the M777 howitzers, according to new reports.
This follows previous waves of training of Ukrainian fighters by the US on similar heavy artillery.
Last week, the US Department of Defense said around 60 percent of the US-made howitzers shipped from America to Ukraine had arrived at their destination.
A US official confirmed the howitzers had been delivered along with 155mm rounds of ammunition.
The official said: “We do believe that these howitzers will be […] very, very effective in helping in the Donbas fight, which we’ve already seen, is deeply reliant on long-range fires, specifically artillery by both sides.
They added: “We certainly believe that the conditions could be set for a much longer slog here inside the Donbas given both sides’ familiarity with the terrain.
“We’ve described it as a potential knife fight, and I think it’s beginning to shape up to be exactly that.”
“And we know that the Russians also believe the same thing, because we’re seeing them move artillery units into the Donbas as well.
“And so we want to give the Ukrainians every bit of advantage that we can.”
Mr Kirby added: “What this provides for the Ukrainian forces is lethal long-range fires that are very effective against everything from lightly armoured vehicles to support vehicles, and in particular the Russian indirect fire system, their artillery and rockets.”
A number of countries have sent M777 howitzer artillery to Ukrainian forces since the Russian invasion began on February 24.
Canadian president, Justin Trudeau, said in a statement in April that the country had “now delivered a number of M777 howitzers and associated ammunition to the Security Forces of Ukraine, in conjunction with our American Allies”.
Days later, Australia committed to providing six M77 howitzers to Ukrainian forces to beat back “Russia’s brutal, unrelenting and illegal invasion,” prime minister Scott Morrison confirmed.
In a joint statement with Australia’s defence minister, Peter Dutton, Mr Morrison detailed: “The $26.7 million in new support for Ukraine takes Australia’s total contribution to Defensive Military Assistance to Ukraine to over $225 million.”
The Australian Army describes the M777 howitzer as a “significant advance in Army’s capabilities”, with a “much higher level of digital connectivity”.
This allows for a “more rapid, safe and accurate application of effects across the battlespace”, according to their website.