British Prime Minister Gordon Brown has said he believes the loss of life in Burma has been "far greater" than that reported by the authorities.
He was speaking after holding talks by phone with US President George W Bush and Chinese Prime Minister Wen Jiabao.
Burmese officials said nine people were killed on Thursday as troops fired tear gas and bullets to disperse crowds of anti-government protesters in Rangoon.
Most internet links have been severed and mobile phone networks disrupted.
''I want to condemn absolutely the appalling level of violence against the people of Burma.
"I am afraid that we believe the loss of life in Burma is far greater than is being reported so far," Mr Brown said.
He said he hoped the combined international pressure from the US, China, the EU and UN would "begin to make the regime see this cannot continue".
Both the British and Australian ambassadors in Burma said they believed the number of dead "could be many multiples" of the number given by state media.
Meanwhile, the White House has confirmed that the UN special envoy, Ibrahim Gambari, will arrive in Burma on Saturday.
"We have called on the Burmese to allow him to be able to meet with anyone he wants to meet, the military leaders, the religious leaders and Aung San Suu Kyi," spokeswoman Dana Perino said.
The BBC's South East Asia correspondent Jonathan Head says Burma's rulers have turned their backs on the world and the torrent of outrage their actions have provoked.
The first opportunity to communicate that outrage will be when Mr Gambari arrives in the country to persuade the generals to put a stop to the crackdown.
The violent suppression of anti-government protesters continued for a third day on Friday, with soldiers and police baton-charging crowds who tried to stage marches.
The security presence in the main city was the heaviest yet seen.
Troops sealed off the key religious sites in Rangoon, including the Shwedagon and Sule pagodas - the focal point of some of the larger protests earlier in the week.
Police also surrounded five monasteries to prevent Buddhist monks - who have been spearheading the demonstrations - from taking to the streets.
All the main roads into central Rangoon have been blocked.
Information from Burma has become increasingly patchy. Internet access has been cut in Rangoon and is only partially available elsewhere.
Burmese sources told the BBC that international mobile phone signals have been interrupted and soldiers are searching people for cameras and mobile phones.
Dissidents have been using the internet to get pictures and video of the protests and the military crackdown to international news outlets - who then fed them back into Burma via the internet and satellite TV.
But eyewitnesses managed to contact the BBC on Friday to say that the government was sending bus-loads of vigilantes into the main city to attack the demonstrators.
They said a temporary prison had been set up at an old race course for the hundreds, possibly thousands, of people detained in recent days.
The atmosphere is said to be extremely tense and there is a palpable sense of fear on the streets.
Correspondents say Rangoon looked like a city locked down on Friday morning but at about 1300 local time (0700GMT) small groups of protesters began gathering.
Some were immediately rounded up. Within minutes, shots could be heard - but it was not clear if they were being fired into the crowds or overhead in warning. A witness told the BBC that a number of people were killed in Friday's violence.
Loudspeaker trucks have been criss-crossing the city, warning people not to protect anyone fleeing arrest.
An overnight curfew for the South Okkalapa district, the scene of Thursday's violent encounters between soldiers and protesters, was also announced.