A typhoon currently off the Philippines may approach southwestern Japan sometime on Wednesday or Thursday, the Meteorological Agency said Sunday.

Typhoon Mitag, the season's 18th typhoon, is expected to gather strength Monday and move closer to Okinawa Prefecture, the agency said, warning of possible strong winds and rough seas.

As of Sunday, the typhoon that formed a day before was traveling west-northwest off the east of the Philippines at a speed of 20 kph (12 mph).