SYDNEY (REUTERS) - A delayed rotation of US Marines to a defence base in Australia's northern city of Darwin will go ahead based on strict adherence to Covid-19 measures, Australia's defence minister said after speaking with her US counterpart.

Up to 2,500 marines had been scheduled to arrive in April, in a major defence alliance cooperation exercise, but this was postponed in March because of the coronavirus pandemic.

The remote Northern Territory, which has recorded just 30 Covid-19 cases, closed its borders to international and interstate visitors in March, and any arrivals must now undergo mandatory quarantine for 14 days.

Defence Minister Linda Reynolds said in a statement that she and US Secretary of Defence Mark Esper had agreed that a modified rotation could proceed, with marines required to undergo the 14-day quarantine and comply with other Covid-19 requirements.

The US rotation may be reduced in size or duration upon advice from health authorities, a spokeswoman for Ms Reynolds said.

Ms Reynolds said the pandemic has "served to reinforce the importance of the alliance between our two nations".

"Our defence organisations' focus is now on maintaining force readiness and helping our partners in the Pacific and South-east Asia," she said.

Australian and US defence science experts, as part of the Five-Nation Research and Development Council, were cooperating on responses to the Covid-19 crisis, Ms Reynolds said, including examining the survival of the virus on surfaces and the impact of temperature and humidity.

The council includes science and technology researchers in the United States, Australia, Canada, New Zealand and Britain, the nations that are also members of the Five Eyes intelligence grouping.