LOS ANGELES, Sept. 9 (Xinhua) -- Typhoon Mangkhut will strike Guam from local time Monday afternoon and it is gaining strength to pose a dangerous threat to southeastern China by Friday or the weekend, the U.S. National Weather Service (NWS) forecasted.

According to the Typhoon Warning for Guam released by the NWS at the Guam local time 7:40 a.m. Monday morning, Mangkhut, whose center was located about 170 miles (287.3 kilometers) east-southeast of Saipan and 245 miles (414.1 kilometers) east-northeast of Guam, is expected to become the equivalent of a Category 2 or 3 hurricane as it rakes through the U.S. island territories in the Western Pacific.

Mangkhut is moving west at 21 mph (35.5 kph) with maximum sustained winds of 90 mph (152.1 kph), the NWS said, adding that it would likely intensify after striking the Marianas and tracking into the open Pacific Ocean on the west of Guam this week.

Interests in the Philippines, southeastern China and Japan's far southwestern Ryukyu Islands hence should monitor the progress of Mangkhut, the report said.

The NWS also warned that combined seas could reach 40 feet (12.2 meters) near the center of the storm, and seas outside the storm center could reach 20 feet (6.1 meters).

Guam's emergency agency urged residents since from last week to prepare for a potentially catastrophic typhoon with widespread damage and extended power outages. All inbound flights for Monday and departures on Tuesday have been canceled.

The last major typhoon to directly hit the island was in 2002 with Supertyphoon Pongsona, which left the entire island of Guam without power, destroyed about 1,300 houses, killed one person and cost 700 million U.S. dollars.