Chairman of the Joint Chiefs of Staff Mark Milley said it was too early to say whether the counter-offensive had failed.

"F-16s. That is moving forward, actually. So there's a training program in place and they'll likely receive F-16s ... in the not-too-distant future," Chairman of the Joint Chiefs of Staff Mark Milley said in an exclusive interview with Jordan's Al-Mamlaka public service TV news channel.

Ukrainian President Volodomyr Zelenskiy recently said he had received pledges from Denmark and the Netherlands to deliver F-16s, saying they would strengthen Ukraine's air defences and help its counter-offensive against Russian forces.

Milley said the counter-offensive faced heavy Russian reinforcements with many months to prepare minefields, tank ditches and dragon's teeth tank obstacles "in a very complex set of defensive preparations that the Ukrainians are fighting through ...

"The Ukrainians have a significant amount of combat power remaining; this is not over yet," he said. "So I think it’s frankly too early to say whether it (has) succeeded or failed."

"Clearly it’s had partial success to date. Now the speed at which the offensive has been undertaking is slower than the planners had thought," Milley added in the interview in Amman.

Ukrainian forces were being trained intensively in various parts of Europe on "command and control of offensive formations and combined arms maneuver and to break complex obstacles that will bolster its capabilities," the four-star U.S. Army general said.

"Again, it's bloody, it's long and it's slow. And we had predicted that several months ago," he said.

Washington was still deliberating providing long-range Army Tactical Missile Systems known as ATACMS that Ukraine wants and which reach behind enemy lines, including Russia, Milley told Al-Mamlaka.

"They've gotten long-range artillery with the Guided Multiple Launch Rocket System (GMLRS) and the Brits have provided other types of capabilities that other countries have as well. So they've gotten a lot of artillery," he said. "But the ATACMS is a controversial topic.

"And for a lot of reasons, they haven't received those yet. They're still on the table. President Biden has not said no or yes at this time," Milley said.

Milley, who is slated to retire later this year, again cautioned against the likelihood of an outright Ukrainian military victory in the near term while highlighting the possibility of a political solution to end the fighting.

"It’s an offensive that is been going on for about, I guess, eight weeks or so. It’s very bloody, slow, high casualty- producing and its very difficult. So the idea of militarily kicking out two or three hundred thousand Russian troops is going to be very difficult and challenging.

"A different way of getting out of this is through negotiations and may be that will happen too."