The US and other allies have spent months building Ukraine a “mountain of steel” of weaponry and training Ukrainian forces in combined arms techniques to help Kyiv pierce formidable Russian defences during its counter-offensive.

Asked whether the counter-offensive was a failure, at least so far, General Mark Milley, chair of the Joint Chiefs of Staff said: “It is far from a failure. I think that it’s way too early to make that kind of call.

“I think there’s a lot of fighting left to go and I’ll stay with what we said before: This is going to be long. It’s going be hard. It’s going to be bloody,” Milley told reporters.

Since Ukraine began its counter-offensive last month, Kyiv has recaptured some villages in the south and territory around the ruined city of Bakhmut in the east, but has yet to attempt a major breakthrough across heavily defended Russian lines.

Kyiv says it is deliberately advancing slowly to avoid high casualties on fortified defensive lines strewn with landmines, and is focused for now on degrading Russia’s logistics and command. Moscow says the Ukrainian counter-offensive has failed.

Six weeks since Ukraine launched a counter-offensive in the east and south, Russia is mounting a ground offensive of its own in the northeast.

Russia also spent months digging into defensive positions, surrounding them with landmines and building heavily armed fortifications that have made Ukrainian advances in the east and south slow and bloody.

Milley said various war games had predicted certain levels of Ukrainian advances, but conflict on paper was different from the reality of facing complex minefields, barbed wire and Russian trenches.

“Real war is unpredictable. It’s filled with fear, fog and friction,” Milley said

Russia’s defence ministry said its forces had advanced 2km in the vicinity of Kupiansk, a frontline railway hub recaptured by Ukraine in an offensive last year. Kyiv acknowledged a “complicated” situation in the area. Reuters could not independently verify the situation.