American officials remain privately frustrated over Australia's decision to donate retired Abrams tanks to Ukraine, even as the vehicles finally begin the long sea journey to the battleground.

Axar.az reports via ABC, during an overnight meeting in Rome, Ukraine's President Volodymyr Zelenskyy thanked Prime Minister Anthony Albanese after he confirmed the fleet of second-hand M1A1 vehicles was on the way.

The publiacion has confirmed that the process of loading the first of the 49 vehicles onto a cargo ship began recently, but the government will not discuss their current location or expected arrival date in Europe for "security reasons".

Before the shipping process could begin, Australia had to wait for Washington's approval to export the US-made tanks to a third country, which was granted despite American officials remaining privately frustrated about the donation.

"Last year, even before Donald Trump returned as president, we warned the Australians that sending these Abrams tanks would be complicated, and once they finally get to the battlefield the Ukrainians will find them difficult to sustain," one US official said.

Last month an Australian Defence official said there were some concerns that sending the large tanks to Ukraine was not the best way to provide military assistance to the war-torn country.

"We are starting to doubt if the Ukrainians actually want these vehicles. The tank roof is the weakest point of the Abrams and this is a drone war," the official noted.

Defence Minister Richard Marles on Monday declined to say whether US officials have expressed any concerns about the donation.

"We've been working very closely with Ukraine, very closely with the United States, to see this shipment occur and to see the tanks be on their way," he told reporters in Melbourne.

"The first tranche has been on their way now for some time but I'm not going to go into the specific details of that."