YANGON (AFP) - Frustrated world leaders tightened the pressure on Myanmar on Saturday, raising the allegation of crimes against humanity over the regime's slow-moving response to the cyclone disaster.

US President George W. Bush extended sanctions on Myanmar while British Prime Minister Gordon Brown denounced the junta's "inhuman" treatment of around two million survivors battling to stay alive two weeks after the storm hit.

With the toll of dead and missing now 134,000, the pressure appeared to mark a shift in tactics in the face of the junta's reluctance to allow a full-scale emergency effort, despite fears of more deaths from hunger or disease.

"We have an intolerable situation created by a natural disaster," Brown, whose country was the colonial power when Myanmar was known as Burma, told the BBC.

"It is being made into a man-made catastrophe by the negligence, the neglect and the inhuman treatment of the Burmese people by a regime that is failing to act and to allow the international community to do what it wants to do."

Wary of any foreign influence that could weaken its 46 years of iron rule in Myanmar, the junta has insisted on managing the operation itself and kept most international disaster experts away.

But aid groups say the government cannot possibly handle the tragedy by itself, with hundreds of tonnes of supplies and high-tech equipment piling up in warehouses, bottle-necked by logistics and other problems.

Nobel Peace Prize winner Desmond Tutu wrote to Brown, Bush and French President Nicolas Sarkozy, calling on the UN Security Council to authorise aid drops over the objections of the generals.

He said the regime had "effectively declared war on its own population and is committing crimes against humanity."

Jean-Maurice Ripert, France's UN ambassador, told a meeting of all members of the United Nations that the situation was turning "slowly from a situation of not helping people in danger to a real risk of crimes against humanity."

Bush announced that sanctions on the junta would be extended for a year because of its "large-scale repression of the democratic opposition." The statement stressed it would not affect US humanitarian cyclone aid.

Faced with the mounting criticism, the junta flew some diplomats and aid workers Saturday into the heart of the disaster zone -- which has been all but sealed off to the outside world.

"What they showed us looked very good," said Chris Kaye, Myanmar director for the UN's World Food Programme, after taking one of the state-run tours. "But they are not showing us the whole picture."

However one diplomat said: "It was like a steam-roller had gone through the entire delta region."

In its report on the visit, state television said more than 5,600 tonnes of relief supplies had been delivered to regions battered by the storm -- and that there was "wrong information" that aid had been restricted or refused.

It said 47 medical specialists from India and another 30 from Thailand arrived on Saturday.

The junta has blocked journalists from getting to the southern Irrawaddy Delta, the rice-growing region hardest hit when Cyclone Nargis hit on May 2-3, bringing powerful winds and massive waves that wiped whole villages away.

But those who have got through have returned with tales of unspeakable misery, including from some survivors who said they had received very little assistance from the government.

Survivors have also reported that the military was pushing them out of temporary shelter in monasteries, whose revered Buddhist monks helped lead massive anti-government protests last year that were eventually put down.

Navy ships from France and the United States are positioned off the Myanmar coast stocked with emergency supplies, but have not been able to enter.

The regime is said to fear a possible invasion by the United States, which has criticised Myanmar for keeping democracy activist Aung San Suu Kyi under house arrest -- and for its slow moves toward elections promised by 2010.

The government said this week that 99 percent of eligible voters had cast their ballots last Saturday in a referendum it said approved a new constitution which would bar her from office.

Her party rejected the result and said the vote should never have been held amid the cyclone tragedy. The regime has scheduled round two of the vote, in the disaster areas, on May 24.