Image copyright EPA Image caption The attack took place as polio workers and security staff were preparing to set off for their immunisation rounds

At least 15 people have been killed in a suspected suicide bombing outside a polio vaccination centre in the south-western Pakistani city of Quetta.

Many of the casualties are thought to have been police guarding the clinic.

Armed guards are routine for polio workers in Pakistan, who have been the target of many deadly attacks by Islamist militants in recent years.

Militants oppose polio vaccination, saying it is a Western conspiracy to sterilise Pakistani children.

Pakistan and Afghanistan are the only two countries where the disease is still endemic.

'People screaming'

The explosion took place in the morning as polio workers and security staff were reporting for duty before heading out on their vaccination rounds, said Sarfaraz Bugti, a minister in Balochistan province of which Quetta is the capital.

Quetta's deputy commissioner, Dawood Khilji, said the death toll had risen to 15, with 14 police officers and one passer-by confirmed dead. About 20 people were injured.

One of the police officers who survived the blast said his team had been preparing to leave for various neighbourhoods around Quetta when they were hit.

"Suddenly there was a loud bang and I fell to the ground, I could not see anything, there was dust everywhere," said Shabir Ahmed, who suffered shrapnel wounds to his stomach, hands legs and feet.

"Then I heard people screaming and sirens of ambulances," he told the AFP news agency.

Mr Khilji told the BBC that the polio drive in the province had not been suspended as a result of the attack.

Analysis: M Ilyas Khan, BBC News, Islamabad

This is one of the bloodiest attacks targeting a polio vaccination campaign in the Balochistan region in recent months.

It comes in the wake of a sharp fall in militant attacks in Pakistan, mainly caused by a ground offensive the military launched in June 2014 to clear the North Waziristan region of Taliban militants. As such, it serves as a stark reminder that door-to-door polio vaccination continues to be a hazardous occupation.

While it is not clear who carried out the attack, Balochistan is torn by an armed separatist conflict, and is said to be home to Afghan Taliban as well as some local armed groups with Islamist leanings who oppose separatists and are therefore known to have backing from some official quarters.

The attack comes amid rising political differences among central and provincial politicians over the details of a $46bn economic corridor that China is funding to link its north-western region with Gwadar, a coastal town in Balochistan. There are fears that attacks like this could jeopardise Chinese investment.

Image copyright AFP Image caption Polio vaccination programmes have led to a dramatic drop in worldwide cases

Pakistan recorded more than 300 polio cases in 2014 - its highest number since 1999. Most of the new infections were in north-west Pakistan, where militants regularly target roving health teams, and health officials blamed the rise in cases on several deadly attacks on police workers that year.

The number of cases fell to just over 50 in 2015, largely because vaccination teams could reach areas that were previously off limits because of militancy.

The last attack on a polio target in Pakistan is thought to have taken place in north-western Swabi district in November 2015, when the local polio co-ordinator was shot dead by unidentified gunmen.

Polio around the world

Image caption By 1988, polio had disappeared from the US, UK, Australia and much of Europe but remained prevalent in more than 125 countries. The same year, the World Health Assembly adopted a resolution to eradicate the disease completely by the year 2000.

Image caption In 2015, polio was endemic in only two countries - Pakistan and Afghanistan. No new cases were reported in Africa.

What is polio?

Polio (poliomyelitis) is a highly infectious disease caused by a virus which invades the nervous system. It mainly affects children aged under five.

Symptoms include fever, fatigue, headache, vomiting, stiffness in the neck and limb pain.

One in 200 infections leads to irreversible paralysis, and between 5-10% of those who suffer paralysis die because their breathing muscles are immobilised.

Cases have fallen dramatically since polio eradication programmes were introduced; from 350,000 globally in 1988 to around 70 in 2015.

Polio remains endemic in Afghanistan and Pakistan, but Nigeria was removed from the list in October after a year with no new cases.