A drought has left areas of the Marshall Islands facing "dire" water shortages with aid agencies scrambling to ship relief to affected communities, officials in the Pacific nation said Wednesday.

With almost no rainfall since late last year on some of the northern islands, the government this week issued a disaster declaration as villages began rationing water to preserve supplies.

"We've got 3,700 people without drinking water, the situation is dire," national water advisor Tom Vance said on Wednesday following a trip to Mejit Island.

Health officials said water tanks were running low and water from wells had turned brackish, making it unsafe to drink. Without rain, the only other source of liquid for the islanders is coconuts.

"The situation is increasingly desperate if this drought persists in the northern islands and atolls," said Chief Secretary Casten Nemra, who is heading the emergency relief program.

The United States and Australian governments this week announced $100,000 in emergency aid grants, with the US also donating four reverse osmosis water-making units which are expected to arrive in the next few days.

The Marshall Islands government said it hoped to buy more of the units and send a ship to the disaster zone carrying 450 collapsible water containers filled to capacity.

Nemra said the drought had severely damaged local food crops and there were health fears for the population, with cases of diarrhoea, conjunctivitis, influenza and other illnesses already being reported.

"The northern Marshall Islands is under incredible level of hardship and reports indicate that conditions will get worse in the coming days," he said.