By Amie Ferris-Rotman

KABUL, April 12 (Reuters) - Gently massaging the soft flesh under his knees, 20-year-old Abdul Ahmat recalls the suicide bomb six months ago that destroyed his legs.

The former construction worker, in a wheelchair after his legs were amputated, is among a growing number of Afghans severely wounded by bombs that have grown grimly more powerful than ever before in three decades of conflict.

"I stepped onto the street to head to work, when suddenly I became helpless. I knew I had lost my legs," the father of one said of the attack that killed 13 foreign troops and four Afghans in the capital, Kabul, in October 2011.

Ahmat, who had come to Kabul from relatively peaceful Bamiyan province in search of work, spoke in a Red Cross orthopaedic centre, one of the largest in the world and one of seven the humanitarian organisation operates in Afghanistan.

The free-of-charge centres log some 6,000 new patients every year, all of them Afghans. Of those, 1,000 are direct victims of war, many grievously wounded by the heightened potency of bombs.

"I can't say that the number of Afghan bomb victims is increasing, but the power of the blasts certainly is," said Alberto Cairo, an Italian physiotherapist in charge of the orthopaedic centres. He started his work in Kabul in 1990.

Bombs have changed over the course of the war, experts say, evolving from crude affairs using cheap explosives packed with nails and other bits of metal into relatively more sophisticated devices using fertilisers imported from Pakistan.

Military officials, who did not wish to be identified, said the Taliban are now increasing the chance of infection among victims by covering bombs with urine, faeces or blood, leading to an increased likelihood of amputation.

"REALLY DEVASTATING"

No statistics exist for Afghan amputees, but the victims of more powerful bombs are yet another example of civilians who have borne the brunt of a war becoming even more dangerous as foreign combat troops prepare to withdraw by the end of 2014.

Civilian deaths in Afghanistan were up for a fifth straight year in 2011, according to the United Nations, with more than 3,000 killed last year alone.

The vast majority were killed by insurgents.

In its report, the United Nations said bombs - also known as improvised explosive devices, or IEDs - planted on roads and increasingly deadly suicide attacks that targeted civilians killed more people than any other form of attack.

"It is really devastating what bombs can now do. In the past we had a few double amputee patients, now we see quite a lot," Cairo told Reuters as he tended to a middle-aged IED victim trying out a new pair of prosthetic legs.

The difference between old and new bombs in the orthopaedic centre is stark: most of the one-legged men are grey-haired and wearing turbans, many victims of Soviet mines during the war with Moscow in the 1980s.

T-shirt clad younger patients, such as one man in his twenties who lost both legs above the knees in December's suicide attack, appear to have suffered graver injuries.

Almost all of the Kabul centre's 323 members of staff are themselves disabled from war or disease, something which the Red Cross says helps boost patient morale. Other patients include children suffering from congenital diseases.

The Kabul centre also has an on-site factory, where disabled workers turn out custom-made artificial legs and arms from individual moulds.

TALIBAN'S DEADLIEST WEAPON

Civilians are not the only victims of increasingly powerful bombs that are also believed responsible for more amputations among U.S. troops.

NATO forces fighting the Taliban say IEDs are the insurgents' deadliest weapon. Those who disarm the bombs are considered to have the most dangerous jobs in the increasingly unpopular war, which has dragged into its eleventh year.

Last year a record number of American soldiers needed amputations, with the number reaching 20 a month, outpacing even the most violent years of the war in Iraq, U.S. reports say.

Smaller IEDs are also being made, again usually from fertiliser smuggled from Pakistan, aimed at passers-by.

For 11-year-old Norzia, the fear of IEDs is very real. The daughter of a shepherd was hit by a Soviet-era landmine when tending to her family's sheep in eastern Wardak province.

Her pretty face framed by a colourful hijab, she gingerly walked on artificial legs towards her physiotherapist, who guided her with a slight limp.

"I want our government to stop the war in our country, I want the Taliban to stop planting roadside bombs. This is what can happen," she said. (Reporting by Amie Ferris-Rotman; Editing by Paul Tait)