SYDNEY, March 9 (Reuters) - Australia's nuclear submarines will ensure peace and stability across the Indo-Pacific, southeast Asia and Indian Ocean, Defence Minister Richard Marles said on Thursday ahead of a landmark agreement between Washington, Canberra and London.

U.S. President Joe Biden will host Australian Prime Minister Anthony Albanese and British Prime Minister Rishi Sunak on Monday in San Diego, site of major U.S. Navy operations, to chart a way forward for Australia's plans to obtain nuclear-powered submarines.

"Clearly, these submarines will have the capability to operate at war, but the true intent of this capability is to provide for the stability and for the peace of our region," Marles told parliament.

The agreement, known as the AUKUS pact and announced in 2021, will provide Australia the technology and capability to deploy nuclear-powered submarines amid China's military buildup in the Indo-Pacific region. China "firmly objects" to AUKUS, its foreign ministry said this month.

"It is difficult to overstate the step that as a nation we are about to take ... we have never operated a military capability at this level before," Marles said. "I want to say, at this moment to our neighbours and to our friends around the world that as Australia invests in its defence ... we do so as part of making our contribution to the peace and the stability of our region and of the world."

AUKUS is expected to be Australia's biggest-ever defence project but it has not been announced whether it will involve a U.S. or a British-designed submarine, or a combination of both.

Reuters, citing four U.S. officials, reported on Wednesday Australia was expected to buy up to five U.S. Virginia-class nuclear submarines in the 2030's as part of AUKUS.

Albanese declined to confirm the report.

"I'll be making further comments about specific proposals at the appropriate time but I can confirm that on Monday there will be a meeting of the AUKUS partners," Albanese, on an official trip in India, told reporters in Ahmedabad.