SYDNEY, July 24 (Reuters) - Australia said on Monday it would spend A$9.8 billion ($6.60 billion) to buy 20 new Super Hercules military transport aircraft, ahead of a visit later this week by U.S. Secretary of State Antony Blinken and Defense Secretary Lloyd Austin.

The U.S. State Department in November approved the potential sale of the aircraft, manufactured by Lockheed Martin (LMT.N), and related equipment to Australia.

The deal comes amid the largest ever Australia-U.S. joint military exercise, involving more than 30,000 troops and participants from 11 other countries, in a show of force and unity at a time when China has emerged as an increasingly assertive power in the Indo-Pacific.

The biennial Talisman Sabre war games began on Friday and will take place in various locations across Australia for two weeks.

Australian Defence Minister Richard Marles and Austin will travel to north Queensland to watch the military exercise following the annual meeting of U.S. and Australian defence and foreign ministers, known as AUSMIN, in Brisbane on July 28-29.

The new C-130J Hercules aircraft will replace and expand upon the aging fleet of a dozen planes currently operated by the Royal Australian Air Force, with delivery of the first expected from 2027, Marles said in a statement.

"From bushfire and flood emergencies across the country, the delivery of crucial supplies to the region during the COVID-19 pandemic and more than two decades supporting peacekeeping operations, this has and will continue to be a crucial asset," Marles said.