U.N. officials, who had earlier complained that an airlift of emergency supplies for the victims was being delayed, said several cargo planes had now landed at Yangon's airport.
WFP spokesman Paul Risley in Bangkok said a Thai cargo plane delivered seven tons of high-energy biscuits and a U.N. chartered flight from Brindisi, Italy arrived in Yangon with water, plastic sheeting, medical kits and other equipment.
He said one other charter flight in Bangkok was awaiting landing clearance permission and a fourth flight was expected to leave from Dubai on Thursday.
The Red Cross/Red Crescent confirmed its first aid plane took off from Kuala Lumpur, carrying six tonnes of shelter materials.
"It's a modest amount, but we hope once we established it, others will follow," an official said. "Another eight tonnes of shelter goods will leave on a Thai commercial flight tonight."
Medicins sans Frontieres, which has 1,000 people in Burma, said it was ferrying aid supplies into the delta via trucks and boats. It said it had been granted permission to fly in supplies.
"We are focusing on those still alive; 50 percent of them have wounds and they are infected," MSF official Frank Smithius in Burma told Australian radio. "Because of the winds and high water, people got smashed around."
Jean-Michel Grand, executive director of Action contra la Faim in London said the logistical obstacles were formidable.
"The roads are very poor or destroyed, and in many cases there were no roads before. Everybody's looking at boats as an alternative. It's going to be a massive logistics challenge.
The WFP's Risley said aid agencies normally expect to fly in experts and supplies within 48 hours of a disaster, but nearly a week after this cyclone, few international groups have been able to send reinforcements into Burma.
Thai Prime Minister Samak Sundaravej failed to reach Burma's ruling generals on Thursday after U.S. President George W. Bush asked him to intervene with the junta to expedite the international aid effort.
"We couldn't reach them because the communication towers have been damaged," Thai government spokesman Wichianchot Sukchotrat told reporters.
State media had reported a death toll of 22,980 with 42,119 missing as of Tuesday, but diplomats and disaster experts said the real figure is likely to be much higher.
"The information that we're receiving indicates that there may well be over 100,000 deaths in the delta area," said Shari Villarosa, charge d'affaires of the U.S. embassy in Burma.
