BEIJING, April 24 (Reuters) - China and Singapore will hold a joint military exercise as soon as this week, their first combined drills since 2021, as Beijing deepens its defence and security ties with Southeast Asia, a region with strong existing U.S. alliances.

The Chinese navy will deploy a missile-bearing frigate, the Yulin, and a mine-hunting ship, the Chibi, to the joint maritime exercise which will last from late April to early May, the Chinese defence ministry said in a statement on its website on Monday, without specifying the location.

Two years ago, China and Singapore held a combined military drill in international waters at the southern tip of the South China Sea, following the upgrade of a bilateral defence pact in 2019 to include bigger-scale exercises among their army, navy and air force.

The deeper China-Singapore military cooperation comes as a time of heightened tensions in the South China Sea, an area spanning 3.5 million square km (1.4 million square miles) that is often traversed by Western navies including U.S. vessels conducting freedom of navigation operations. Such passages annoy China, which lays claims to nearly all of the South China Sea despite an international ruling to the contrary.

In August last year, the U.S. military conducted an expanded Super Garuda Shield exercise with Indonesia that saw the participation of Singapore, Japan and Australia for the first time.

Around the same time, China sent fighter-bombers to Thailand in joint air force drills code named Falcon Strike 2022. Both countries say the exercises, in northeast Thailand near the border with Laos, were defensive in nature.

The drills last summer also took place against the backdrop of elevated tensions in the Taiwan Strait following the visit of former U.S. House Speaker Nancy Pelosi to the democratically governed Taiwan, which China claims as its own.

China's increased military engagement in Southeast Asia is widely expected to challenge the influence that the United States has shaped with countries including Singapore and Indonesia in coming years.