The United States on April 13 unveiled a $800-million tranche of equipment for Ukraine, including helicopters, howitzers and armored personnel carriers.

"There have been four flights from the United States arriving into the theater just yesterday," a senior Pentagon official said Monday, with a fifth flight due shortly.

The package includes 18 155mm howitzers for the first time, as well as 40,000 artillery rounds, 200 M113 armored personnel carriers, 11 Mi-17 helicopters and 100 armored multi-purpose vehicles.

Many modern armaments use electronic chips whose main producers are Taiwan and South Korea, two US allies that have stopped exporting products to Russia in line with American sanctions.

"There has been an effect on Putin's ability to restock and resupply, particularly in the realm of components to some of his systems and his precision-guided munitions," the US official said.

Russia has formally complained to the United States over its military aid to Ukraine, warning of "unpredictable consequences" if shipments of advanced weaponry go forward, US media reported last week.