By Jacob Gronholt-Pedersen

COPENHAGEN (Reuters) - Ukrainian President Volodymyr Zelenskiy told a crowd in Denmark on Monday that promised deliveries of U.S.-made F-16 fighter jets had made him confident Ukraine could end Russia's invasion.

Denmark and the Netherlands on Sunday announced they would supply the first F-16s to Ukraine, with the initial six due to be delivered around New Year. Washington had approved the delivery of the jets ahead of Zelenskiy's trip to Copenhagen.

"Today we are confident that Russia will lose this war," Zelenskiy told thousands of people who gathered outside the Danish parliament to hear his speech.

Russia warned earlier that supplying the jets to Ukraine would only escalate the war, which has dragged on for almost 18 months. Russian forces are occupying almost a fifth of Ukraine's territory and Kyiv is battling hard to push them out.

"The fact that Denmark has now decided to donate 19 F-16 aircraft to Ukraine leads to an escalation of the conflict," Russian ambassador Vladimir Barbin said in a statement cited by the Ritzau news agency.

"By hiding behind a premise that Ukraine itself must determine the conditions for peace, Denmark seeks with its actions and words to leave Ukraine with no other choice but to continue the military confrontation with Russia," he said.

The Ukrainian military said the jet was vital to the success of its counteroffensive, which has proceeded slowly since its launch in early June, as it would prevent Russian fighter jets attacking advancing forces.

"Superiority in the air is key to success on the ground," air force spokesman Yuriy Ihnat was quoted as saying by Ukrainian media.

Danish Defence Minister Jakob Ellemann-Jensen said Ukraine may only use the donated F-16s within its own territory.

"We donate weapons under the condition that they are used to drive the enemy out of the territory of Ukraine. And no further than that," Ellemann-Jensen said on Monday.

"Those are the conditions, whether it's tanks, fighter planes or something else," he said.

Denmark will deliver 19 jets in total. The Netherlands has 42 F-16s available in all but has yet to decide whether all of them will be donated.

Ukrainian President Volodymyr Zelenskiy has called the decision a "breakthrough agreement".

Defence Minister Oleksiy Reznikov said on Saturday that Ukrainian pilots had begun training, but it would take at least six months and possibly longer to also train engineers and mechanics.

(Reporting by Jacob Gronholt-Pedersen and Pavel Polityuk; Editing by Angus MacSwan and Philippa Fletcher)