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.

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.

"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."

Austin said the Dutch and Danish defense ministers are working with the U.S. 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," Austin said, adding that the training is "an important example of our long-term commitment to Ukraine security."

Austin earlier in the day 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.