Residents in the Marshall Islands capital Majuro attempted a clean up Saturday after a powerful storm damaged houses and smashed boats while another unseasonal typhoon brewed nearby.

Majuro Atoll's normally calm lagoon was turned into a cauldron of high waves on Friday, ripping fishing vessels and yachts from their moorings and smashing them on to reefs.

High winds tore roofs from houses, knocked down trees and caused a power outage affecting half the city which is home to about 25,000 people.

Local officials said the severity of the storm was unusual because July is outside of the typhoon months for this north Pacific nation.

"We've been here through many westerlies, but I have never seen anything quite so ferocious," said Cary Evarts, an American who has lived aboard his yacht in Majuro for more than 15 years.

He estimated 25 vessels in the lagoon either broke loose or dragged their moorings and Majuro's five-kilometre (three-mile) long eastern lagoon coastline was littered with debris from boats and damaged houses.

"We've seen inundation and damage all along the shoreline," said longtime local fisherman and Sea Patrol officer Kyle Aliven. "This is the worst I've seen."

Meanwhile, the United States Joint Typhoon Warning Center issued a "typhoon formation alert" for the Marshall Islands, saying a typhoon was building between Majuro and Kwajalein, the two most populated atolls in the western Pacific nation.