Britain has launched an operation to airlift 6,000 Egyptian refugees stranded on the Libyan-Tunisian border back to their home country.

Updating MPs on the British response to the humanitarian situation, David Cameron said there were "serious indications of a growing humanitarian crisis", with some 162,000 people reported to have crossed the land borders so far.

The prime minsiter, who faced criticism last week for the pace of Britain's response to evacuate Britons caught up in the Libyan uprising, surprised MPs by with the airlift operation.

He said technical teams from the Department for International Development had been sent to Libya's borders with Egypt and Tunisia, and on Tuesday the UK government had flown in tents for 1,500 people and blankets for 36,000.

The first flight for the airlift from the Tunisian-Libyan border was scheduled to leave the UK later on Wednesday, Cameron said.

Britain will use three chartered plans, flying in rotation, to evacuate 6,000 Egyptian citizens. The planes will fly from Britain to Djerba, in Tunisia. They will then embark on a series of flights from Djerba to Cairo.

Britain is sending the flights after receiving a request from the UN, which has warned that 85,000 people are stranded on the border.

Cameron told MPs: "It is vital to do this. These people shouldn't be kept in transit camps if it's possible to take them back to their home, and I'm glad that Britain can play such an important part in doing that."

Ed Miliband, the Labour leader, had asked what support was being offered to international organisations helping to tackle the crisis.

Cameron said HMS York has now docked in the eastern Libyan port of Benghazi, carrying medical and other supplies that would help with the humanitarian mission.

The prime minister also told MPs he was still committed to carrying out preliminary work on establishing a no-fly zone over Libya.

Asked by Miliband about the unease across the world about imposing a no-fly zone, the prime minister said: "We should, and we are, looking at plans for a no-fly zone. I was particularly heartened by what Secretary of State Clinton said. She said a no-fly zone is an option we actively considering. These issues are being discussed at the North Atlantic Council, and I think it is right that they are."

Cameron also made it clear Britain wants to see Muammar Gaddafi overthrown. He said: "I think we should support, and say how much we admire, those brave people who are standing up in their own country, asking for greater freedoms, greater democracy – the things we take for granted in our own country.

"Many ... said any sort of rebellion like this would either be extremist or Islamist, or tribal. It is none of those things. It is revolt by the people who want to have greater democracy in their country."

British sources, who admit that military intervention is unlikely at the moment, indicated that action would be taken if Gaddafi used the several tonnes of mustard gas that are under his control.

One source said: "We know he has stocks of mustard gas. They are in a secure compound. They are not weaponised. As part of the deal he reached on his WMD, he agreed to reduce them; the question is whether he sticks by his commitments."

In a speech carried on state TV in Libya, Gaddafi repeated his determination to "fight to the last man and last woman, again claiming popular support for his regime as forces loyal to the Libyan dictator battled to wrest cities from control of his opponents.

Gadaffi blamed al-Qaida for the 15 days of unrest, and warned thousands of people would die if the US and Nato invaded.

"I carried out a revolution in the 70s, handed over power to the people and then rested, " he told a loyal audience in Tripoli opening a rambling speech covered by state TV.

Gaddafi said the UN could send fact-finding committees to prove his forces had not fired on peaceful civilians, and said there had been "no protests at all in the east".

He said reports of deaths were exaggerated, putting the toll at 150. Other estimates say up to 2,000 people have died.

Responding the decision by Britain and other countries to freeze his assets, Gaddafi said: "I'll put my fingers in Cameron's eyes if I keep assets abroad." His also salary was only 465 dinars (£232).

Gaddafi's defiance came as both sides claimed to control the city of Brega, about 150 miles from Tripoli. Ajdabiya, nearly 50 miles from Brega, has reportedly been targeted by Gaddafi's airforce.