Prime Minister David Cameron on Thursday paid a pre-Christmas visit to British troops serving in Afghanistan, insisting that the "high price" paid by servicemen had been worthwhile, his office said.

Cameron told reporters that Afghan security forces were "doing better than expected" as he defended plans for the British force to be cut to 5,200 next year, according to his office.

The premier, who was touring Camp Bastion in Helmand Province, said he was confident that the draw-down of nearly half of the British troops deployed, announced in parliament on Wednesday, would not pave the way for an increase in terrorist activity.

"The fact is they (Afghan security forces) are doing better than expected," he said.

"This is withdrawal. This is draw-down based on success, not on failure."

He added that the decision was taken "for good military reasons and it has been done in a proper way.

"We're confident it can be done while making sure Afghanistan does not return to become a haven of terrorism which is of course why we came here in the first place," stressed the prime minister.

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.

Brigadier Bob Bruce, commander of Task Force Helmand, backed the prime minister's comments, saying standards had "risen markedly" over the last year.

"The Afghans working with us now really are in control," he told the BBC. "The insurgency is still there. It's not gone but it doesn't dictate things."

Cameron joined in a carol service and played table-football with soldiers during Thursday's flying visit. He also announced �230 million ($365 million, 283 million euros) in extra funding for equipment.

This will go toward providing additional Improvised Explosive Device (IED) detectors, more military working dogs and an upgrade to armoured vehicles.

The leader claimed that Afghanistan was now a "far better place than it was when we came here in 2001", but admitted it remained a "deeply challenged country".

"We have paid a very heavy price but I think the reason for coming here in the first place, which was to stop Afghanistan being a haven for terror... I think it was the right decision," he explained.

He said on Wednesday that 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 added.

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