Agencies warn the death toll could rise to 1.5m unless aid is delivered
Britons should keep donating cash to Burma's cyclone victims despite the ruling junta blocking help from abroad, Douglas Alexander has insisted.
The international development secretary said donations were "already making a difference" thanks to UK charities.
Burma's military regime says it will accept foreign aid but not admit overseas aid workers into the country.
Brendan Gormley, of Disasters Emergency Committee, said aid that was getting through was making a "huge difference".
According to the Disasters Emergency Committee (DEC), UK donations so far have totalled £5m.
The official death toll has risen to almost 30,000 but aid agencies fear 1.5m could die if help does not come.
Speaking to the BBC, Mr Alexander admitted that he would like to see aid reaching victims more quickly, but said donations were already helping.
"The money is getting through to the charities," he said. "And the charities themselves, as much as they can, are already working on the ground to save lives.
"It's not getting in as quickly we would like, but all the money goes to accredited British charities and it's already making a huge difference to those people to whom we can actually get that aid."
Brendan Gormley, DEC chief executive, added that the Burmese regime was allowing aid through.
He said: "As aid channels are established, it is even more important that the funds are available to continue the flow of emergency relief supplies.
"Thousands in Burma are in desperate need of water, food and medical supplies. Please give to the DEC Appeal so we can meet the challenge."
Mr Alexander's intervention came as the first US aid flight to Burma following the cyclone landed in Rangoon.
The US spent days negotiating with Burma's military government to gain permission for the aircraft to land.
The first aircraft from medical relief agency Medecins Sans Frontieres (MSF) also arrived in the country early on Monday.
Conservative leader David Cameron has said it may soon be necessary to start dropping relief supplies by air.
Aid agencies estimate that 100,000 have already perished, but an official from Save the Children in Burma told the BBC that about half the people affected by the cyclone were receiving help.
The UN, which has launched a $187m (£96m) appeal for aid, says people urgently need food, water, shelter and medical aid.
Many victims are said to be dehydrated or suffering from injuries that have not been treated.