The American defense secretary, Lloyd Austin, said Thursday that European allies are developing a coordinated program to train Ukrainian forces on the F-16 fighter jet, but Pentagon leaders warned that it will be a costly and complex task and won’t be a magic solution to the war.

Mr. Austin said the allies recognize that in addition to training, Ukraine will also need to be able to sustain and maintain the aircraft and have enough munitions. And he said air defense systems are still the weapons that Ukraine needs most in the broader effort to control the airspace.

“There are no magic weapons,” the chairman of the joint chiefs of staff, General Mark Milley, who spoke alongside Mr. Austin at a Pentagon press conference, said. Providing 10 F-16s could cost $2 billion, including maintenance, he said.

“The Russians have a thousand fourth- and fifth-generation fighters, so if you’re going to contest Russia in the air, you’re going to need a substantial amount of fourth- and fifth-generation fighters.”

As a result, he said, allies did the right thing by first providing Ukraine with a significant amount of integrated air defense to cover the battlespace. He said F-16s have a future role as part of Ukraine’s air capabilities, but it’s “going to take a considerable length of time to build up an air force that’s the size and scope and scale that would be necessary.”

In the meantime, overnight on Thursday Russia launched missiles at Kyiv — the 13th such attack on the capital so far in May. No casualties were reported. A rocket attack early Friday at Dnipro reportedly caused more than a dozen injuries and at least one death.

Also on Friday, Russia’s deputy security council chairman, Dmitry Medvedev, dismissed any prospect of negotiations with Ukraine as “impossible” for as long as President Zelensky is in office. “Everything always ends in negotiations, and this is inevitable, but as long as these people are in power, the situation for Russia will not change in terms of negotiations,” Mr. Medvedev said, according to the TASS news agency.

Mr. Austin said the Dutch and Danish defense ministers are working with America on the effort, and that Norway, Belgium, Portugal, and Poland have already offered to contribute to the training. In addition, he said the allies will set up a fund so that other nations can contribute to the overall effort.

“We expect more countries to join this important initiative,” Mr. Austin said, adding that the training is “an important example of our long-term commitment to Ukraine security.”

Mr. Austin said he hopes that training for Ukrainian pilots on American-made F-16 fighter jets will begin in the coming weeks, bolstering Ukraine in the long run but not necessarily as part of an anticipated spring counteroffensive against Russia.

Mr. Austin and General Milley spoke at the close of a virtual meeting of defense leaders from around the world to discuss the ongoing military support for Ukraine. Ukrainian leaders gave them an update on the war effort and the military gaps that troops are facing. Mr. Austin said the biggest gap continues to be ground-based air defense.

The leaders, in their 12th meeting, heard about ongoing combat operations and the counteroffensive and discussed how the allies, who have faced their own stockpile pressures, can continue to support Kyiv’s fight against Russia. Ukrainian officials have not formally announced the launch of their much-anticipated counteroffensive, though some say it has already begun and the pace of attacks suggests it’s under way.

European allies have been vocal in their support for the fighter jet training in recent days.

The European Union’s foreign policy chief, Josep Borrell, said Tuesday that training for Ukrainian pilots had begun in Poland and some other countries, though the Polish defense minister, Mariusz Blaszczak, said training was still in the planning phase. The Netherlands and Denmark, among others, are also making plans for training.

“We can continue and also finalize the plans that we’re making with Denmark and other allies to start these trainings. And of course, that is the first step that you have to take,” the Dutch defense minister, Kajsa Ollongren, said.

Ukraine has long sought the sophisticated fighter to give it a combat edge as it battles Russia’s invasion, now in its second year.

The Biden administration’s decision was a sharp reversal after refusing to approve any transfer of the aircraft or conduct training for more than a year because of worries that doing so could escalate tensions with Russia. U.S. officials also had argued against the F-16 by saying that learning to fly and logistically support such an advanced aircraft would be difficult and take months.