Pakistani authorities have arrested five men said to have links with extremist groups for their purported involvement in a May 13 ambush involving members of a minority community.

In the attack 45 people were killed and about 12 injured when six gunmen said to be Sunni extremists attacked a bus.

Four suspects who figured in the brutal attack against Ismaili Shia Muslims riding a bus in the Pakistani capital were taken into custody by the police, according to a top provincial official.

"Four militants, who planned the horrific attack on the bus, have been arrested in Karachi," Sindh Chief Minister Qaim Ali Shah told reporters at Nawabshah district of Sindh province on May 20, ucanews reported.

Ismailis are Shia Muslims who revere Imam Ismail a prophet who died in 765 AD. They number about 15 million worldwide with some 500,000 living in Pakistan.

"Statements from witnesses have been investigated and verified and soon all the attackers will be put behind bars," he continued.

The fifth suspect, said to be the mastermind behind the attack, had been arrested in Bahawalpur district of Punjab province, intelligence sources told the media.

Reports indicated that authorities swiftly dispatch a security team to Bahawalpur to check out the suspect and bring him to Karachi for questioning.

Prime Minister Nawaz Sharif expressed his gratitude to law enforcement agencies and the provincial chief minister over the arrests, said to have been swifter than usual.

The prime minster praised the swift action, praising the authorities.

"The arrest of plotters of this heinous crime in such a short time proves that Karachi is going in the right direction and that our law enforcement agencies have the capability of handling such challenging tasks," Sharif said.

The six gunmen boarded the bus on which the victims were passengers, mowing them down.

The attack drew worldwide condemnation from the United Nations, the United States, Europe, and human right groups.