WASHINGTON, June 23 - The United States will send another $450 million in military aid to Ukraine, including some additional medium-range rocket systems, to help push back Russian progress in the war, officials announced Thursday.

The latest package includes four High Mobility Artillery Rocket Systems, or HIMARS, which will double the number they have now, The Associated Press reported. All four were prepositioned in Europe, and training on those systems has already begun with the Ukrainian troops who will use them, said Marine Corps Lt. Col. Anton Semelroth, a Pentagon spokesman. The first four HIMARS that the U.S. previously sent have already gone to the battlefield in Ukraine and are in the hands of troops there.