A severe drought in the northern Marshall Islands has prompted the government to declare a state of emergency.

President Christopher Loeak approved the decision after receiving a recommendation by the government's disaster preparedness committee.

Marshall Islands meteorologist Regi White has told Radio Australia's Pacific Beat a tropical low over the northern islands has severely impacted rainfall.

"Wotje Atoll, one of the islands most to the north of Marshall Islands, has experienced zero rainfall on several of the previous months," he said.

"Usually this is the drier season for Marshall Islands - especially the northern islands - but this year the tropical low has decided to dig down further to the south and that has caused a little bit more severe than normal conditions."

Many of the northern islands depend almost entirely on rain for fresh water for the local populations, which range from less than 100 to more than 500.

Regi White says while the islands are used to dry conditions, this year has been particularly difficult.

"Rain drinking water and wild waters have become salty," he said.

"Usually the capacity of the water reservoirs...for drinking water only...could last for three or four months - we have heard that they're almost empty.

"We're hearing reports from the local government mayors in the northern islands that certain species of food crops have begun to wither."

Under the emergency declaration, government officials will be dispatched to drought-affected areas.

Maintenance crews are also expected to be sent to the islands to examine reverse osmosis equipment - designed to purify water supplies - which are reportedly not operating at full capacity.

Last month, the United States donated two new reverse osmosis machines - which are able to produce 1,800 litres of potable water a day.