WASHINGTON - Hurricane Irma, a powerful storm, will soon threaten much of the eastern Caribbean, the US National Hurricane Center said Monday, with several islands under hurricane warnings and Puerto Rico and the Virgin Islands under watches. Irma, now a category three hurricane with maximum sustained winds of 120 mph (195 kph), should continue strengthening through Tuesday, the NHC said in an advisory issued at 1500 GMT. Some forecasters expect it to reach category four strength — the maximum attained by Hurricane Harvey, which recently devastated parts of Texas and Louisiana — by that time.