* Rain hampers aid effort

* Aid deliveries minimal so far, agencies say

* Calls for firm action on aid

* Myanmar's slow response "callous," says Australia's Rudd

(Updates with calls for aid action)

By Aung Hla Tun

YANGON, May 13 (Reuters) - Heavy rains pelted homeless cyclone survivors in Myanmar's Irrawaddy delta on Tuesday, complicating the already slow delivery of aid to more than 1.5 million people facing hunger and disease.

As more foreign aid trickled into the former Burma, critics ratcheted up the pressure on its military rulers to accelerate a relief effort that is only delivering an estimated tenth of the supplies needed in the devastated delta.

In Brussels, the European Union called on the military junta to allow entry to aid workers to help victims avert "an even greater tragedy," and France urged U.N. action if the junta did not cooperate. Spain said that failure to allow aid in could amount to a crime against humanity.

The United Nations says more than 1.5 million people are struggling to survive and up to 100,000 are dead or missing after cyclone Nargis hit.

U.N. spokeswoman Elisabeth Byrs said in Geneva it was also vital to secure the means to deliver aid.

"We need a kind of air bridge or sea bridge, and huge means (just) as the aid delivery we did in the tsunami, it is the same kind of logistical operation," said Byrs, of the U.N. Office for the Coordination of Humanitarian Affairs.

The junta has accepted aid from the outside world but the help has only trickled in as the rulers have made it clear they do not want outsiders distributing it.

In a statement after emergency talks on Myanmar in Brussels, EU development ministers called on Yangon "to offer free and unfettered access to international humanitarian experts, including the expeditious delivery of visa and travel permits."

The EU ministers stopped short of endorsing a French call to deliver aid if necessary without the junta's permission.

France's junior minister for human rights said it had the backing of Britain and Germany to call on the U.N. Security Council for aid to be taken into Myanmar without the government's green light if necessary.

"We have called for the 'responsibility to protect' to be applied in the case of Burma," Rama Yade told reporters.

British officials said London would welcome discussion of the responsibility to protect but did not consider the proposal realistic at present given Russian and Chinese objections.

PLANES ARRIVING

An Australian air force plane landed in Yangon, Myanmar's main city, with 31 tonnes of emergency supplies, a day after the first U.S. military aid flight arrived in a country Washington has described as an "outpost of tyranny."

Two more U.S. flights arrived on Tuesday as part of a "confidence-building" effort to prod Myanmar's reclusive generals into allowing a larger international relief operation 11 days after the disaster.

Tens of thousands of people throughout the delta are crammed into Buddhist monasteries and schools after arriving in towns that were poor even before the disaster.

Lacking food, water and sanitation, they face the threat of diseases such as cholera. The heavy rains added to their misery.

"Where I am now, there's over 10,000 homeless people and it's pouring rain," Bridget Gardener of the International Red Cross said during a rare tour of the delta by a foreign aid official.

While a steady stream of aid flights have landed in Yangon, only a fraction of the relief needed is getting to the delta due to flooding and the junta's desire to keep most foreign aid and logistics experts either out of the country or in Yangon.

The World Food Programme said it was able to deliver less than 20 percent of the 375 tonnes of food a day it wanted to move into the flooded delta.

Myanmar state television said six ships carrying 500 tonnes of supplies had left Yangon for the delta on Tuesday.

International relief organizations say their local staff are stretched to the breaking point, while Medicins Sans Frontieres said its workers faced "increasing constraints."

At the United Nations in New York, U.N. officials are worried some of the aid might have been diverted to people who were not cyclone victims but have no hard proof that has occurred, spokeswoman Michel Montas said.

"That concern exists," she said, noting that "a very small percentage of victims have so far received the aid."

One Yangon businessman who returned from a personal aid mission to Bogalay, a delta township where at least 10,000 people were killed, told Reuters that soldiers were appropriating aid.

"There are still some villages in the worst-hit areas that nobody has got to," the man, in his late 30s, said. "Around Bogalay, private donors are not allowed to distribute their assistance to the victims themselves. We had to hand over what we had." (Additional reporting by Carmel Crimmins in Bangkok; David Brunnstrom and Ingrid Melander in Brussels; Writing by Peter Millership; Editing by Philip Barbara)

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