The United States aims to break the dynamic of grinding warfare and near-frozen front lines in Ukraine with newly announced military capabilities that it hopes will breath fresh momentum into Kyiv's battle against Russian forces, a senior Pentagon official said on Wednesday.

But the official, Colin Kahl, who just returned from a visit to Kyiv, said the Pentagon still wasn't prepared to meet Kyiv's calls for Abrams tanks.

"I just don't think we're there yet," Kahl said. "The Abrams tank is a very complicated piece of equipment. It's expensive. It's hard to train on. It has a jet engine."