Relief for those affected by Cyclone Nargis has been slow to arrive
Prime Minister Gordon Brown says he has asked the United Nations to organise an emergency summit on the situation in Burma in the wake of Cyclone Nargis.
At Commons question time, Mr Brown said there had been an "improvement" in the number of relief planes being allowed in, but it was still "not good enough".
Tory leader David Cameron joined the PM in urging the Burmese regime to let more emergency aid in.
He asked Mr Brown for a deadline after which aid drops would be enforced.
"Of course on direct aid, of course the prime minister is right that the best step is to get the government to open up the country, to allow the aid agencies in but I do think it is worth setting a deadline by which we say enough isn't through - and more should be done," Mr Cameron told MPs.
"Yes of course, the experts say that with direct aid only a fifth of it will get through, but a fifth of something is better than nought per cent of nothing."
Mr Brown said the UK was exerting diplomatic pressure on Burma and although he did not rule out aid drops, aid agencies believed this was still the best way to help.
Mr Brown said he has asked the UN to convene an emergency summit and Foreign Office minister Lord Malloch-Brown had gone to Asia to talk to ministers there about coordinating action.
"They key thing at the moment is to pressure the regime by all countries in Asia uniting with all of us to make sure that aid gets to the people of Burma as quickly as possible.
"We are ready to do everything in our power. I hope the whole House will unite in saying that the Burmese regime must now let into the country all aid workers and all aid immediately."
He said the natural disaster in Burma had been turned into a "natural catastrophe" by the "actions of a despicable regime".
'Responsibility to protect'
But, he added, the "key thing for all of us is to get aid to the people of Burma as quickly as possible by the means available to us."
Asked later which member of the UN security council had vetoed enforced aid drops, Mr Brown said he did not want to "name names".
Earlier, Foreign Secretary David Miliband said the "responsibility to protect" UN doctrine, drawn up to apply in cases like genocide, could apply to natural disasters.
"All instruments of the UN should be available," he told the BBC.
Meanwhile, the United Nations said it had only been able to reach 270,000 of the 1.5m survivors of Cyclone Nargis.
It called for an air or sea "corridor" to be opened up to channel large amounts of aid to the region and warned of the risk of a "second catastrophe" if a major operation did not begin.
Some European nations have called for the UN principle of "responsibility to protect" to be applied, which would allow aid deliveries without Burma's consent.
Asked about the issue for Tuesday's BBC Radio 4's World Tonight programme, Mr Miliband said the problem with the protocol - drawn up to protect people against war crimes, genocide, ethnic cleansing and crimes against humanity - was that it was a "legal requirement".
But asked if it could apply in the event of a natural disaster, he said: "It certainly could and we have been absolutely clear in New York over the last 12 days that all instruments of the UN should be available."
However, asked if aid convoys could be sent into Burma with military escorts, without the military regime's permission, he said "you could try to" but pointed out there were 400,000 Burmese troops in uniform.
"No-one should think that there is an easy or quick answer to this," he said.
He said "all options are being looked at" - pointing out that HMS Westminster had been sent to the area, alongside French and US military assets.
The military junta that rules Burma has admitted that parts of the worst-affected region remain cut off, but the generals are still refusing entry to foreign aid workers.
However, the first US aid flight landed in Rangoon on Monday, after days of negotiation.