A hurricane and a tropical storm are heading west across the Pacific Ocean toward the tourist haven of Hawaii and the U.S. National Hurricane Center said parts of the islands may need to post watches later on Tuesday.

Hurricane Iselle was about 1,700 kilometres east-southeast of Hawaii, moving west at 15 km/h with maximum sustained winds of 200 km/h, the National Hurricane Center said.

"Watches may be required for portions of the islands by later today or early Wednesday," the Miami-based weather forecaster said in an advisory.

Hurricane Iselle

Hurricane Iselle is moving closer to Hawaii. (National Hurricane Center)

Residents stocked up on basics as authorities in Honolulu advised them to prepare a seven-day disaster supply kit. The hurricane was expected to turn west-northwest later on Tuesday and Wednesday, the National Hurricane Center said, but was forecast to weaken over the next 48 hours.

Further east over the Pacific, Tropical Storm Julio was about 1,845 kilometres from Baja California in Mexico and also expected to continue moving west-northwest through Thursday, the NHC said.

Shoppers in Honolulu waited in line at supermarkets with carts full of bottled water, batteries and nonperishable food items.

That storm was moving at 20 km/h and has maximum sustained wind speeds of 95 km/h.

The Honolulu Department of Emergency Management has urged residents to prepare for emergency needs for as long as seven days without basic resources, and shoppers in the city waited in line at supermarkets with carts full of bottled water, batteries and nonperishable food items.

"With Hawaii's remoteness, it could be as long as a week before a full disaster relief operation can be initiated," the department said in a statement late on Monday.

Hawaii normally safe from storms

Hurricanes rarely hit Hawaii. The state was washed over by Hurricane Flossie in 2007, which caused 6 metre waves but very little damage. Hurricane Neki did minor damage to a marine national monument in the northwest of the islands in 2009.

In 1992, Hurricane Iniki pummeled the island of Kauai, killing six people and causing estimated damages of $2.4 billion. Other major storms including Uleki, Estelle and Iwa have also struck the islands in the 1980s.

Separately on Tuesday, the NHC said Bertha, the second hurricane of the 2014 Atlantic season, had weakened to a tropical storm some 765 kilometres west of Bermuda.