(CNN) -- Logistical challenges have kept help from reaching much of Myanmar's cyclone-ravaged Irrawaddy delta region, relief agencies reported Friday.
Workers for the U.N. children's agency UNICEF have had no trouble with Myanmar officials, UNICEF spokesman Mike Bociurkiw said.
They've reached "far-flung areas," but they must travel hours by boat since most roads and bridges are impassable after Cyclone Nargis roared across Myanmar four weeks ago, Bociurkiw said.
Just one of 10 helicopters pledged for delivery of food, medicine and aid workers has arrived in the country, Bociurkiw said.
Chris Webster, a spokesman for the Christian charity World Vision, said his agency has seen "an opening up" by Myanmar officials as the ruling junta promised U.N. Secretary-General Ban Ki-moon when he visited last weekend.
"We are getting access," Webster said. "Of course, it will need to be tested over a longer period with more aid, more supplies and more staff, but it does appear things are opening up." Watch what's happened to schools in Myanmar »
World Vision has about 500 workers in Myanmar, also known as Burma, delivering shelter, food, water and health care -- which Webster said has reached about 250,000 people in eight townships so far.
UNICEF estimated that about 50 percent of the affected regions have gotten some help.
Staffing is a major challenge, Bociurkiw said. "It's a huge operation" to distribute water purification tablets and vaccines, he said.
Monsoon rains fall daily across the delta region, adding to the challenges, he said.
Human Rights Watch criticized Myanmar's secretive government for refusing to allow U.S., French and British ships waiting offshore with helicopters and small boats to assist.
"Don't let those ships sail away while people continue to suffer needlessly because of the paranoia of the generals," said Brad Adams, Asia director at Human Rights Watch. "There's a race against time to save lives, so the Burmese government needs to let these boats dock now and grant immediate travel to the delta."
In addition, a UNICEF official said the agency is checking into unconfirmed reports that the government is forcibly removing people from temporary camps and leaving them near their home villages with few or no supplies.
About 130,000 people either died or are still missing since the cyclone made landfall on May 2, Ban said Sunday. Myanmar's government has put the death toll at 78,000.
Ban spent last weekend in Myanmar and guided a conference of 52 donor nations Sunday. At the meeting, countries pledged more than $100 million to help Myanmar recover. They said they are willing to open their wallets further once aid groups are granted access to the worst-affected areas.
The Myanmar government had asked for $11 billion in assistance. It said the relief phase of the disaster already is over and that it needed the money for reconstruction and rehabilitation efforts.
But donor nations insisted that immediate relief needs have yet to be met, with Ban saying it will take at least another six months.
CNN's Alan Duke and CNN Radio's Barbara Hall contributed to this report.