Ukraine's counter-offensive against Russia will be difficult and "very bloody", the US' highest-ranking military officer has said.

Gen Mark Milley said he was unsurprised that progress had been slower than predicted - but added that Ukraine was "advancing steadily".

"It goes a little slow, but that is part of the nature of war," he said.

It comes as Volodymyr Zelensky accused "some" Western partners of delaying promised training for Ukrainian pilots.

Several Western countries have pledged to train Kyiv's pilots on US-made F-16 fighter jets, but the Ukrainian president said some allies had been "dragging their feet" on the promise.

President Zelensky has previously acknowledged that the Ukrainian offensive was making slow progress.

Gen Milley, chairman of the Joint Chiefs of Staff, told an audience at the National Press Club in Washington on Friday that the counter-offensive was "advancing steadily, deliberately working its way through very difficult minefields... 500m a day, 1,000m a day, 2,000m a day, that kind of thing".

He added he was not surprised that progress had been slower than expected. "War on paper and real war are different. In real war, real people die," he said. 

"Real people are on those front lines and real people are in those vehicles. Real bodies are being shredded by high explosives.

"What I had said was this is going to take six, eight, 10 weeks, it's going to be very difficult. It's going to be very long, and it's going to be very, very bloody. And no-one should have any illusions about any of that."

Ukrainian soldiers were "assaulting through minefields and into trenches", he said, adding that "this is literally a fight for their life".

He said the US was giving Ukraine "as much help as humanly possible".

Gen Milley is the principal military adviser to the president, the secretary of defence, and National Security Council.

Meanwhile, Ukraine's military commander-in-chief Valery Zaluzhny said the counter-offensive had been hampered by a lack of adequate firepower.

In an interview with the Washington Postpublished on Friday, he said he was frustrated by the slow deliveries of weapons promised by the West, from modern fighter jets to artillery ammunition.

"I do not need 120 planes. I'm not going to threaten the whole world. A very limited number would be enough," he said.

Separately, the head of the US Central Intelligence Agency, William Burns, is reported to have made an unannounced visit to Ukraine last month where he met President Zelensky and Ukrainian intelligence officials.

The CIA director is said to have discussed Ukraine's counter-offensive against Russian forces, as well as reaffirming the US commitment to intelligence-sharing.