Saudi Arabia says troops from 20 countries are gathering in the oil-rich nation for large-scale military exercises.

The kingdom announced the exercises Sunday on the official Saudi Press Agency, describing them as "the largest and most important" military maneuvers in the region's history.

It says the exercises in the country's north will include air, sea and land forces. They are expected to last 18 days, with participating forces arriving in "the next few hours." Most of the participants are Arab and African countries, and do not include the United States and other Westen powers.

Saudi Arabia leads a coalition fighting Shiite rebels in Yemen. It is also part of the U.S.-led coalition against Islamic State, and recently said it is willing to send ground troops to Syria.