Kyiv's forces are facing heavily fortified Russian positions that include complex minefields, tank obstacles, barbed wire and trenches, Chairman of the Joint Chiefs of Staff General Mark Milley told journalists when asked if the counteroffensive had stalled.

Ukraine has "a significant amount of combat power not yet committed," he said.

"Right now, they are preserving their combat power, and they are slowly and deliberately and steadily working their way through all these minefields, and it's a tough fight."

Milley said it was "way too early" to judge the offensive a failure, adding that "this is going to be long, it's going to be hard, it's going to be bloody."

US Defense Secretary Lloyd Austin, speaking alongside Milley at a news conference following a virtual meeting of Ukraine's international backers, said efforts to boost Kyiv's forces are far from finished.

"I would remind you that... this is not over, we continue to generate combat power," with three brigades currently being trained and equipped in Germany and "other training ongoing around the around the region," he said.

"Our work continues, and we're going to do everything we can to make sure that the Ukrainians can be successful," Austin added.

The United States has spearheaded the push for international support for Ukraine, quickly forging an international coalition to back Kyiv after Russia invaded and coordinating aid from dozens of countries.

Washington has agreed to provide more than $42 billion in military aid to Kyiv since the Russian invasion began in February 2022.