The first of three helicopters sent from Britain is starting to distribute aid to victims of the earthquake in Pakistan and Kashmir.
Pakistan needs helicopters to help remote communities
The helicopters are being sent out to reach survivors cut off in isolated mountain villages.
They were put on C17 transport planes at RAF Fairford, in Gloucestershire, on Saturday, and flown to Islamabad.
The aircraft can carry 54 people or at least 25,000lb (11,340kg) of freight. RAF rescue teams are working with them.
A reconnaissance team is already in the region to assess how the RAF can help.
Rescue teams from 27 Squadron, based at RAF Odiham in Hampshire, are accompanying the helicopters.
Defence Secretary John Reid said the RAF assistance comes after the UK sent civilian helicopters as well as ration packs, winter tents and blankets in the two weeks since the earthquake on 8 October.
"The earthquake there was obviously terrible and the aftermath is even worse in an area where travel and transportation are normally hugely difficult," he said.
Dr Reid said Liberal Democrat calls for Chinooks in Afghanistan or Iraq to be sent to Pakistan did not take into account the fact that they were there "protecting the lives of our soldiers and are operationally necessary".
Nato has agreed to send between 500 and 1,000 soldiers to help the earthquake relief effort in Pakistan.
Dr Reid said the UK would be prepared to send troops through Nato but stressed there were not "standing armies either at a British or a Nato level".
Ian Howard-Williams, a programme officer from the Department for International Development, said the RAF's work would be overseen by the UN and the government of Pakistan.
"The most urgent task will be to move forward with shelter items, with some food, according to the priorities on the ground," he said.
The earthquake hit Pakistan, Afghanistan and northern India on 8 October, wiping out several villages in Kashmir and killing more than 50,000 people.
The weather could pose problems, said squadron leader Abrahams
Visiting Pakistan-controlled Kashmir on Friday, International Development Secretary Hilary Benn announced another £20m in UK aid - taking the total to £33m.
He warned of a second wave of deaths if more help failed to reach the devastated region quickly enough.
Squadron leader Mark Abrahams, one of the helicopter crewman travelling to Pakistan, told the BBC approaching winter weather conditions could pose difficulties.
"We are aware that the weather has precluded some of the humanitarian flights already," he said.
"But our crews are trained to take on the challenges of flying in the mountains and we have just returned from a mountain flying detachment so all the crews that will be going are ideally placed to undertake these missions."