WASHINGTON, June 13 (AFP) - NATO Secretary General Jens Stoltenberg told US President Joe Biden at the White House on Tuesday (Jun 13) that Western-backed Ukraine is "making progress" in its offensive to push Russia back from occupied territories.

"Ukrainians are making progress, are making advances," he said in the Oval Office.

Stoltenberg, visiting ahead of July's NATO summit in the Lithuanian capital Vilnius, gave insight into how the powerful Western military bloc sees Ukraine's attempt to turn the tables on Russia.

Ukrainians have talked optimistically about their long-prepared counter-attack across fronts in the east and south eventually driving Russian troops from the entire country.

Stoltenberg emphasised the operation as a way to gain negotiating leverage.

"The more land the Ukrainians are able to liberate, the stronger hand they will have at the negotiating table," he said.

Biden welcomed Stoltenberg, who is due to step down at the end of his term in October, saying that NATO's reaction to the Russian invasion of Ukraine had made the alliance stronger.

"We've strengthened NATO's eastern flank, made it clear we'll defend every inch of NATO territory. I say it again: the commitment of the United State to NATO's Article V is rock solid," Biden said, referring to the pledge by members to come to each other's defence.

"At our summit in Lithuania next month, we're going to be building on that momentum," Biden said.

Earlier, Stoltenberg told CNN that it was "still early days" for the offensive, which he characterised as "difficult".

Meanwhile, the US State Department announced a new US$325 million package of ammunition and heavy weaponry to top up Ukraine's supplies as its big attack gets underway.

The Pentagon said the package provides "key capabilities to aid Ukraine's efforts to retake its sovereign territory and support Ukraine's air defenders as they bravely protect Ukraine's soldiers, civilians, and critical infrastructure".