Typhoon Mawar is bearing down on the US territory of Guam in the Pacific packing ferocious winds and could hit the island on Wednesday, forecasters said.

As of 0900 GMT Monday, the storm was just under 500 kilometers (300 miles) from Hagatna, the capital of the island of 170,000 people, said the US Joint Typhoon Warning Center.

"Heavy rainfall is likely to develop tonight through the next few days. Rainfall amounts of 8 to 15 inches are possible with locally higher amounts. A slowing forward speed could lead to much higher rainfall totals," Guam's Joint Information Center said.

"Mawar is a real threat and a possible direct hit to our island," Guam's governor Lou Leon Guerrero said.

"It's been quite some time since we've had a storm of this magnitude and it is frightening. I ask you to remain calm. Stay informed. And most importantly, be prepared."

Authorities urged people to hunker down and stock up on food, water, medicine and first aid supplies.

Around 30 flights from or to Guam on Tuesday and Wednesday have been scrapped, according to A.B. Won Pat international airport in Guam.

The island is a key strategic location for the United States in the Pacific, hosting a large military presence. It also has a significant tourism industry.