The new $550 million package will "include more ammunition for the high mobility advanced rocket systems otherwise known as HIMARS, as well as ammunition" for artillery, National Security Council spokesman John Kirby told reporters.

The assistance includes 75,000 rounds of 155 mm artillery ammunition, a statement from the Pentagon said.

"To meet its evolving battlefield requirements, the United States will continue to work with its allies and partners to provide Ukraine with key capabilities," the statement said.

This brings the total of military assistance committed to Ukraine since President Joe Biden took office to more than $8.8 billion, according to the Pentagon.

Previous weapons assistance from Washington to Kyiv has included counter-artillery radars, Javelin anti-tank missiles, Soviet-made helicopters, shells and light armored vehicles.

Kirby also hailed the first shipment of Ukrainian grain that left from the port of Odessa on Monday.

It was the first since the war began, part of a landmark deal brokered by the United Nations and Turkey to lift Moscow's naval blockade in the Black Sea.

"We obviously welcome this important step and we hope to see more ships depart in the coming days to travel onward to world markets with agricultural products such as grain, wheat, sunflower oil and corn," Kirby told reporters.

The five-month halt of deliveries from war-torn Ukraine -- one of the world's biggest grain exporters -- has contributed to soaring food prices, hitting the world's poorest nations especially hard.

Russian troops invaded Ukraine on February 24 and the war has left thousands dead, forced millions to flee their homes and raised fears of a nuclear disaster.