Russia has failed to achieve its major goals in Ukraine's Donbass region, but has made "incremental tactical successes" there, the top US general said on Wednesday.

Addressing reporters at the Pentagon, Chairman of the Joint Chiefs of Staff Mark Milley said Russian forces have "failed to achieve their strategic and operational objectives" in Ukraine's industrial heartland, despite months of strikes aimed at occupying and ultimately annexing the region. Moscow began its "special military operation" in Ukraine on Feb. 24.

Asked by a reporter if Donbass, which includes the breakaway regions of Luhansk and Donetsk, has been lost to Russia, the top US general said "it's not lost yet."

"The Ukrainians are making the Russians pay for every inch of territory that they gain," he added.

That is in part due to Ukrainian forces using recently-received High Mobility Artillery Rocket System (HIMARS) long-range rocket systems from the US, which Milley said Kyiv's forces have been using "effectively" to strike target critical to Russia's command and control operations, logistical networks, field artillery and air defense sites.

"These strikes are steadily degrading the Russian ability to supply their troops, command and control their forces, and carry out their illegal war of aggression," said Milley.

Defense Secretary Lloyd Austin said the strikes being carried out on Russia's supply lines will "create dividends" for Ukraine's armed forces going forward.

There are already 12 HIMARS transferred to Ukraine from the US, with an additional four on their way to the war-torn nation. Hundreds of rockets for the system have also been supplied, and 200 Ukrainian forces have been trained on the systems, said Milley.

Also complicating Russia's war effort are what Milley described as "very effective resistance networks" established by Ukraine to carry out covert attacks in territories occupied by Russia.

"The Russians are challenged not only to their front with the Ukrainian conventional forces, but they're also challenged in their rear areas. Their rear areas are not secure for sure," he said.

The US says Iran is preparing to transfer hundreds of drones, including some capable of offensive operations, to Russia to aid in its war effort, and Austin warned Tehran against becoming further involved.

"We would advise Iran to not do that. We think it's a really, really bad idea. And I'll leave it at that," he said.