YANGON (Reuters) - Myanmar's junta has given the U.S. military permission to fly in relief supplies for the survivors of Cyclone Nargis, Thai Supreme Commander Boonsrang Niumpradit told Reuters on Thursday.

"We have helped the Americans to talk to the Myanmar government to allow U.S. planes participating in Cobra Gold to fly humanitarian aid to Myanmar. They just agreed," he said, referring to joint U.S.-Thai military exercises.

A U.S. embassy official confirmed the decision and Boonsrang said the first flights could leave Thailand within a day or two.

"They were very suspicious that the Americans would do more than just distribute relief supplies, but we helped convince the Burmese to allow the Americans in," Boonsrang said.

The decision is a surprise given the huge distrust and acrimony between the former Burma's generals and Washington, which has imposed tough sanctions to try to end decades of military rule.

However, international pressure had been building on the junta to throw its doors wide open to an international relief operation for the worst cyclone to hit Asia since 1991, when 143,000 people were killed in neighbouring Bangladesh.

Aid has been trickling into one of the world's most isolated and impoverished countries, although experts feared it would be too little to cope with the aftermath of Nargis, which left up to 100,000 feared dead and one million missing.

The storm pulverised the Irrawaddy Delta with 190 kmh (190 MPH) winds followed by a massive tidal wave on Saturday.

Three planes loaded with vital U.N. emergency supplies for Myanmar's cyclone victims were delayed on Thursday, awaiting clearance from the military government hours after they were due to land, U.N. officials said.

"They need assistance today. They needed it yesterday," Tony Banbury, Asia regional director of the U.N. World Food Programme WFP, said in Bangkok.

"They can't wait and they shouldn't be asked to wait until tomorrow and it's crucial that food, water, shelter and medical supplies need to go in right away."

Another WFP official said the three planes were waiting on tarmacs in Bangkok, Dhaka and Dubai with 38 tonnes of supplies.

The WFP officials said they believed one Thai commercial cargo plane had landed in Yangon with seven tonnes of high-energy biscuits.

CLUTCHING TREES

Cyclone Nargis, which means "daffodil" in Urdu, slammed into coastal towns and villages in the rice-growing delta southwest of Yangon on Saturday, the most devastating storm to hit Asia since 1991, when 143,000 people were killed in neighbouring Bangladesh.

WFP spokesman Paul Risley said aid agencies normally expect to fly in experts and supplies within 48 hours of a disaster, but nearly a week after the Myanmar cyclone, few international relief groups have been able to send reinforcements into Myanmar.

Witnesses reported that villages were destroyed and people fought for survival by clutching trees as the storm brought walls of water charging inland from the sea.

Aid has been trickling in from other Asian nations, but governments and relief agencies are putting increasing pressure on Myanmar's reclusive military rulers to throw their borders wide open to as much help as possible.

Reports of cyclone damage in a country that used to be the world's largest rice exporter added to worries about tight global supplies of the grain.

The U.N. Office of the Coordination of Humanitarian Affairs said 5,000 square km (1,930 square miles) of the delta were under water.

The government insists it has enough rice reserves, although the U.N.'s Food and Agriculture Organisation said damage to crops and storage buildings in the delta could mean that Myanmar will need short-term imports and miss its 2008 export targets.

(Additional reporting by Grant McCool in Bangkok; Editing by Bill Tarrant)