Britain will withdraw almost half of its 9,000 troops from Afghanistan next year as local security forces become increasingly able to take responsibility, Prime Minister David Cameron said Wednesday.

The announcement comes as NATO prepares for a full security handover at the end of 2014, despite fears that a civil war could follow, and amid a spike in "insider attacks" on foreign troops by Afghans in uniform.

Cameron told parliament that the withdrawal of around 3,800 British troops by the end of 2013 was possible "because of the success of our forces and the Afghan National Security Forces".

Britain has the second largest force in Afghanistan after the United States and has lost 438 troops in Afghanistan since the operation began to topple the Taliban in October 2001 following the 9/11 attacks.

"We'll be able to see troops come home in two relatively even steps in 2013-2014, leaving probably around 5,200 troops after the end of 2013 compared with the 9,000 that we have now," Cameron said.

He said there was no final decision on how many troops would stay in Afghanistan after the end of combat operations in December 2014 but said some would remain to help return equipment to Britain and to deal with logistics.

Britain would also honour its commitment to help set up an officer training academy for the Afghans, as well as contributing military assistance and aid programmes, he said.

"We will not be leaving Afghanistan in terms of our support and our help for the Afghans," Cameron said.

The British prime minister added that he was also personally leading efforts to bring Afghanistan and Pakistan together, with Islamabad's cooperation in any future peace in Afghanistan seen as crucial.

Defence Secretary Philip Hammond said the withdrawal of the 3,800 troops would start in April 2013 with further reductions in September or October after the summer fighting season.

"Everybody talks of an increasing confidence, of an increasing competence and an increasing willingness to engage by the Afghan forces -- a step change in the level of what they are able to do," Hammond said.

The final decision on the latest withdrawal was taken at a meeting of Britain's National Security Council on Tuesday, the Ministry of Defence said.

Afghan defence ministry spokesman General Zahir Azimi declined to comment in detail but said Afghan troops were ready to take over.

"We are completely ready to provide security for any parts of the country evacuated by the foreign forces," he told AFP.

Richard Stagg, the British ambassador to Kabul, said on Twitter that the troop drawdown "does not change our commitment to Afghanistan, which is unwavering. It reflects the progress Afghanistan is making."

Cameron discussed the plans with US President Barack Obama in an hour-long video call on Tuesday night during which they agreed the handover plan was "on track", Downing Street said in a statement.

"This would present further opportunities for ISAF countries to bring troops home next year and they agreed to stay in close touch as detailed plans develop," it said, referring to NATO's International Security Assistance Force in Afghanistan.

The US military currently has about 66,000 troops on the ground as part of a NATO-led force of roughly 100,000.

On Wednesday 500 British troops were due to arrive home in Scotland to complete the previously announced reduction from 9,500 to 9,000 that the government had pledged to complete by the end of 2012.

The British announcement came as Kabul laid out a five-step plan that could bring hardline Taliban Islamists into government as efforts to broker peace accelerate ahead of the withdrawal of Western troops.

Concerns that any Taliban return to power could threaten human rights gains are increasingly being outweighed by fears that failure to include them could see a return to the bloody civil war that blighted Afghanistan in the 1990s.