Ukraine has sidelined US-provided Abrams M1A1 battle tanks in its fight against Russia, in part because drone warfare has made it too difficult for them to operate without detection, US military officials say.
The US agreed to send 31 Abrams to Ukraine in January 2023 after an aggressive monthslong campaign by Kyiv arguing that the tanks, which cost about $US10 million ($A15 million) apiece, were vital to its ability to breach Russian lines.
But the battlefield has changed substantially since then, notably by the ubiquitous use of Russian surveillance drones and hunter-killer drones.
Those weapons have made it more difficult for Ukraine to protect the tanks when they are quickly detected and hunted by Russian drones or rounds.
Five of the 31 tanks have already been lost to Russian attacks.
The proliferation of drones on the Ukrainian battlefield means “there isn’t open ground that you can just drive across without fear of detection,” a senior defence official told reporters Thursday.
The official spoke on the condition of anonymity to provide an update on US weapons support for Ukraine before Friday's Ukraine Defence Contact Group meeting.
For now, the tanks have been moved from the front lines, and the US will work with the Ukrainians to reset tactics, said Joint Chiefs of Staff Vice Chairman Admiral Christopher Grady and a third defence official who confirmed the move on the condition of anonymity.
“When you think about the way the fight has evolved, massed armour in an environment where unmanned aerial systems are ubiquitous can be at risk," Grady sadi in an interview this week, adding that tanks are still important.
News of the sidelined tanks comes as the US marks the two-year anniversary of the Ukraine Defence Contact Group, a coalition of about 50 countries that meets monthly to assess Ukraine's battlefield needs and identify where to find needed ammunition, weapons or maintenance to keep Ukraine's troops equipped.
Recent aid packages, including the $US1 billion ($A1.5 billion) military assistance package signed by President Joe Biden on Wednesday, also reflect a wider reset for Ukrainian forces in the evolving fight.
The US is expected to announce Friday that it also will provide about $US6 billion ($A9.2 billion) in long-term military aid to Ukraine, US officials said, adding that it will include much sought after munitions for Patriot air defence systems. The officials spoke on condition of anonymity to discuss details not yet made public.
The $US1 billion package emphasised counter-drone capabilities, including .50-calibre rounds specifically modified to counter drone systems; additional air defences and ammunition; and a host of alternative, and cheaper, vehicles, including Humvees, Bradley Infantry Fighting Vehicles and Mine Resistant Ambush Protected Vehicles.
The US also confirmed for the first time that it is providing long-range ballistic missiles known as ATACMs, which allow Ukraine to strike deep into Russian-occupied areas without having to advance and be further exposed to either drone detection or fortified Russian defences.
While drones are a significant threat, the Ukrainians also have not adopted tactics that could have made the tanks more effective, one of the US defence officials said.
During its recent withdrawal from Avdiivka, a city in eastern Ukraine that was the focus of intense fighting for months, several tanks were lost to Russian attacks, the official said.
A long delay by Congress in passing new funding for Ukraine meant its forces had to ration ammunition, and in some cases they were only able to shoot back once for every five or more times they were targeted by Russian forces.
