A suicide bomber from the banned Pakistani Taliban blew himself up near a patrolling police truck carrying security personnel to protect polio workers in the restive Balochistan province on Wednesday, killing at least four people and injuring 26 others, including 23 policemen.

The attacker targeted the paramilitary Constabulary truck in Quetta's Baleli area, which was going to provide security to polio workers at the ongoing drive.

Quetta Deputy Inspector General of Police (DIGP) Ghulam Azfar Mahesar confirmed the attack and said that a total of three vehicles were hit in the blast.

"Looking at the crime scene and given that the truck toppled, it is estimated that 25 kilograms of explosives were used," he said.

The death toll from the targeted suicide attack in Quetta rose further as another child died during treatment, Geo News reported.

The attack killed a cop and a child on the spot, and reportedly left 26 including 23 cops wounded, it said.

The death toll climbed to 4 after a woman and her child injured in the bombing succumbed to their injuries later in the day, the report quoted the hospital administration as saying.

Earlier, the hospital administration had said that most of the injured are out of danger.

The banned Tehreek-e-Taliban Pakistan (TTP) group claimed responsibility for the attack which came just two days after the group announced to call off its ceasefire.

It said the attack was in retaliation for the killing of Abdul Wali also known as Omar Khalid Khurrassni killed in Afghanistan in August.

Mahesar said that the explosion was a suicide attack as they have found the remains of a suicide bomber near the crime scene.

Preliminary police investigations showed that a rickshaw hit the police truck. Authorities have declared an emergency in the hospitals of Quetta to treat the victims.

Prime Minister Shehbaz Sharif condemned the incident and directed the authorities to initiate an investigation into the attack.

According to the state-run Radio Pakistan, the premier expressed grief and sorrow over the loss of life.

He also said that polio workers across the country were fulfilling their responsibilities without caring for their lives.

"Eliminating polio virus from the country is amongst the top priorities of the government and we will not rest until polio is completely eradicated," he said, asserting that "evil elements" would always fail to harm the anti-polio campaign in the country.

President Dr Arif Alvi also condemned the attack and prayed for the forgiveness of the deceased and the recovery of the injured.

He said that children were Pakistan's most valuable asset and the government was determined to protect them from diseases such as polio.

"The State will not allow anti-social elements to interfere in the mission of complete eradication of polio," he said.

Former premier and PTI chief Imran Khan also expressed deep grief on the attack and prayed for the speedy recovery of the injured people.

Attacks on workers of the anti-polio vaccination drives in Pakistan have increased in recent times.

In March this year, gunmen in northwestern Pakistan shot and killed a female polio worker as she was returning home after taking part in the country's latest anti-polio campaign.

In January last year, armed gunmen shot dead a police officer guarding a team of polio vaccine handlers in northwestern Pakistan.

Pakistan, along with Afghanistan, remains the last polio-endemic country in the world, according to the World Health Organisation.

PTI