Spanish Prime Minister Pedro Sanchez pledged the EU's "unequivocal" support for Ukraine in Kyiv on Saturday as Spain assumed presidency of the bloc, while Ukrainian President Volodymyr Zelensky hit out at foot-dragging over pilot training by "some" western nations.

Sixteen months into Russia's invasion, Ukraine says it is fighting "fierce" battles as part of its counteroffensive launched last month after weeks of anticipation. 

But both Zelensky and his commander-in-chief Valery Zaluzhny registered their frustration over the slow deliveries of weapons and lack of clarity over pilot training. 

Sanchez visited Kyiv on the first day of Spain's EU presidency, saying this "demonstrates a clear and unequivocal political commitment" to Ukraine's bid to join the 27-nation bloc.

It came ahead of a key NATO summit in Vilnius later this month that is expected to map out the future relationship between Ukraine and the Western military alliance.

"In the next few months, we will make progress on issues that are essential for the future of the European Union, yet Ukraine is going to be at the centre of all our debates," Sanchez told reporters during a joint news conference.

He said the European Commission had recently issued an interim assessment on the status of Ukraine's candidacy to join the bloc. "The report is positive and shows significant progress," Sanchez added.

Sanchez and Zelensky released a joint declaration stating that Ukraine's candidacy will be a priority for Spain's EU presidency. The declaration also said that "Spain supports strengthening NATO's partnership with Ukraine, including through the creation of a NATO-Ukraine Council."

Ukraine received EU candidacy status a year ago and is hoping to begin formal negotiations this year on what it needs to do to firm up its membership bid. 

Kyiv also said this week that the time had come for NATO to clarify its stance on Ukraine's membership.

Zelensky thanked Sanchez for his visit and his support. 

"It is extremely symbolic that this visit takes place on the very first day of the Spanish presidency of the EU," he tweeted.

But the Ukrainian president sounded testy when a journalist asked him about Western plans to send F-16 fighter jets to Ukraine.

"I have the same questions you just asked. The same questions to our esteemed partners," he said.

"Do they have an understanding of when Ukraine can get the F-16?" he said. "There is no schedule of training missions. I believe that some partners are dragging their feet. Why are they doing it? I don't know."

Ukrainian forces have claimed limited gains in a highly touted counteroffensive to win back territory that Russia captured after launching its full-scale offensive in February last year.

- 'Pisses me off' - 

Ukraine's military commander-in-chief Zaluzhny also expressed frustration, saying in an interview published Friday that his country's counteroffensive plans have been limited by the lack of adequate firepower, from modern fighter jets to artillery ammunition.

Zaluzhny told The Washington Post he was frustrated by the slow deliveries of promised weaponry from the West.

It "pisses me off" that some in the West complain about the slow start and progress to the long-awaited push against Russian occupying forces in the country's south, he said.

Zaluzhny said his Western supporters would not themselves launch an offensive without air superiority, but Ukraine is still awaiting F-16 fighters promised by its allies.

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

He also complained he has a fraction of the artillery shells that Russia is firing.

- 'A little slow' -

Zaluzhny said he was in constant contact with Western partners, such as Pentagon Joint Chiefs Chairman General Mark Milley, who are keenly aware of his needs.

But Milley alone can't make the decision, and the delays are deadly, Zaluzhny said.

"It's just that while that decision is being made, in the obvious situation, a lot of people die every day -- a lot. Just because no decision has been made yet."

Speaking later Friday in Washington, Milley said the United States and allies were working hard to supply Ukraine. 

"We are giving them as much help as humanly possible," he said.

Milley said the United States was still in talks on providing Ukraine with F-16s and ATACMS, precision missiles that could more than double the range Ukraine's forces are able to target.

He acknowledged that some people had expressed impatience with the pace of the counteroffensive.

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

bur-as/lcm