Pentagon spokesman John Kirby retracted his statement from Tuesday that Ukraine had received fighter jets from an unnamed ally, after weeks of speculation that a neighboring country might provide Russian-made MiG-29s to add to the Ukrainian air force's capabilities against the Russian invaders.

While fixed-wing aircraft have been offered by an unidentified country to bolster Kyiv's defenses, "they have not received whole aircraft from another nation," Kirby told reporters.

"I was mistaken. They have not received whole aircraft from another nation," Kirby said of his Tuesday claim.

"That said, Ukrainians have received through United States coordination and provision enough spare parts and additional equipment such that they have been able to put in operation more fixed-wing aircraft in their fleet than they had even two to three weeks ago," he said.

Separately, a senior US defense official said the parts supply has enabled Ukraine to add 20 previously inoperable jets to its active fighter fleet.

Since early in the war which began on February 24, Ukraine has asked for more aircraft, especially MiG fighters which its pilots are trained to operate.

Three NATO countries -- Poland, Bulgaria and Slovakia -- have those jets in their fleets.

But the NATO alliance has been cautious, concerned it would be seen as taking a direct role in the fight against Moscow's forces and risking expanding the conflict beyond Ukraine's borders.

Meanwhile the senior defense official said 14 US howitzers that Washington said last week would be provided to Ukraine were now being delivered to the region, along with ammunition for them.

Around 50 Ukrainians are being trained outside of Ukraine to operate them, the official said.

"They will get trained on how to use the howitzers and then they'll be able to go back in to Ukraine and train their colleagues," the official said.