British rescue workers have arrived in Pakistan and begun pulling survivors to safety after the earthquake thought to have killed at least 19,000 people.
Two aid flights have left the UK and the government has pledged £100,000, plus medics, staff and aid workers.
A UK team has helped rescue five people from collapsed flats in Islamabad.
The Queen sent her "heartfelt sympathy" and Tony Blair said the number of UK people with roots in the area made the tragedy "even harder to bear".
"We stand ready to provide more assistance, as and when required," said the prime minister in a message to Pakistan's President Pervez Musharraf.
The Queen said she was "shocked" to hear of the devastation caused by the earthquake.
'Race against time'
Britons looking for missing relatives have begun arriving in the region, while thousands in the UK await news.
And Pakistan High Commission has set up visa services at three UK airports.
John Holland, from rescue charity Rapid UK, said workers felt hopeful after finding two survivors within hours of arriving.
"We've got a feeling there's more people alive in here, so it's a race against time now."
"We also have stocks in Lahore, in Pakistan itself, that we can call upon if they're required"
International development secretary
He said they would start looking for two children after their mother and father were released from the rubble on Saturday night.
Their parents believed they had been talking to them up to a few hours ago, he said on Sunday morning.
"We are going to search that area first with specialised equipment and CO2 sniffers and snake-eye cameras, and then obviously we are going to move through the rest of the building," Mr Holland said.
International Development Secretary Hilary Benn said the UK would send more aid to Pakistan.
"We have further stocks of blankets and tents, sleeping mats available in Dubai and we are now talking urgently to the authorities in Pakistan to find out where those are needed, and if they are we will release them and get them there as quickly as possible.
"We also have stocks in Lahore, in Pakistan itself, that we can call upon if they're required."
The UK search and rescue team flew from Heathrow on Saturday, and a second flight left East Midlands airport, Nottingham, carrying staff and supplies such as blankets, bound for the air force base in Islamabad.
Also in the region is a 14-strong team from the Scotland-based International Rescue Corps.
Spokesman John Ryan said the team had temporarily split into three sections, with an advance party of five already en route by helicopter to Pakistan-administered Kashmir.
The team would be assessing the situation and determining what the search areas would be, before another party followed on Sunday evening, and a third - carrying the bulk of equipment and supplies - on Monday, he said.
He said that following advice from local officials they were likely to be heading for the hard-hit rural areas of the region.
Foreign Secretary Jack Straw said the government's initial pledge of help could be increased.
"We will monitor the situation very carefully and if more is needed - either in terms of money or in terms of staff - that will be sent," he said.
British charities have been organising their response to the disaster, with Oxfam GB, Unicef UK, British Red Cross, Save the Children, Muslim Aid and the Kashmir International Relief Fund already having launched appeals.
The Disasters Emergency Committee, an umbrella organisation of some 13 agencies, will meet to discuss the disaster on Monday.
Family 'lost everything'
London-based relief agency Muslim Aid said it had made £100,000 immediately available.
Sir Iqbal Sacranie, secretary general of the Muslim Council of Britain, said: "We are sure that British Muslims will be generous in sending aid to the affected regions, especially in this holy month of Ramadan."
"It's because people are desperate to find out if their relatives are OK," he said.
Ishfaq Ahmed, chief executive of the Kashmir International Relief Fund, London, said the feeling among Kashmiri communities in Britain was "total devastation".
One man living in Scotland said his wife, two children and extended family in Abbottabad, around 75 miles (120km) north west of Islamabad, had survived but lost their house, land and orchid farm - the family's source of income.
They fled to stay with relatives in Rawalpindi, but have been so traumatised they have found it difficult to talk on the phone, he said.
"This thing scared the hell out of them. My father is head of the whole family, I have never seen him in such a state. Yesterday he could not even talk to me.
"It is so difficult to be here alone. I have no idea what they are going to do, at the moment I am just trying to console them."
Pakistan's High Commissioner in London, Dr Maleeha Lohdi, told the BBC it was advising against travel to the area to allow relief efforts to get under way.
The 7.6-magnitude quake hit Pakistan, Afghanistan and northern India at about 0850 local time (0450 BST), wiping out several villages in Kashmir and injuring more than 42,000 people.
"Four of my housemates are from Muzaffarabad and every single one of them has lost a loved one in this tragic earthquake"