QUETTA - A suicide bomber killed at least 128 people, including a candidate for provincial assembly, and injured more than 150 Friday in Mastung, in the day's second attack on a political rally as fears of violence rise ahead of July 25 polls.

The Islamic State claimed the bombing, according to group's Amaq propaganda agency.

The blast came hours after another bomb killed four people at a campaign rally in Bannu.

The attacks underscored the fragility of country's dramatic improvement in security and raised fears ahead of the vote.

"Death toll has risen to 128," Balochistan Health Minister Faiz Kakar told the media, adding there were more than 100 wounded.

According to senior provincial official Saeed Jamali, the bomber detonated in the middle of a compound where a political meeting was taking place. Another senior official, Qaim Lashari, also confirmed it had been a suicide blast.

The explosion killed Siraj Raisani, who was running for a provincial seat with the newly formed Balochistan Awami Party (BAP), Balochistan Home Minister Agha Umar Bungalzai told AFP.

"Mir Siraj Raisani succumbed to wounds while he was being shifted to Quetta," he added. Raisani was the younger brother of former provincial chief minister Mir Aslam Raisani.

Qaim Lashari, the deputy commissioner of Mastung, told local media that a suicide bomber exploded his explosives-laden vest near the convoy of Nawabzada Siraj Raisani as it reached a BAP rally in the Darengarh area of Mastung.

Most of the injured were shifted to the Civil Hospital Mastung while those in critical condition had been sent to the Civil Hospital in Quetta for better treatment where an emergency has been declared with additional medical staff summoned, according to local media reports.

President Mamnoon Hussain and caretaker Prime Minister Nasirul Mulk strongly condemned the deadly attack in the district. Both of them expressed deep grief and sorrow over the loss of lives in the attack and directed the concerned authorities to provide best medical treatment to the injured.

The Election Commission of Pakistan has announced to postpone elections in the constituency owing to the death of Raisani.

Following the series of attacks, activists called for authorities to remain vigilant to protect candidates during the final days of the campaign season.

"The authorities have a duty to protect the rights of all Pakistanis during this election period - their physical security and their ability to express their political views freely, regardless of which party they belong to," said Omar Waraich, deputy South Asia director at Amnesty International.

The military has warned of security fears in the run-up to the tense election on July 25, and said it will deploy more than 370,000 soldiers on polling day. Militants have targeted politicians, religious gatherings, security forces and even schools in Pakistan. But security across the country has dramatically improved since government and military operations cleared large swathes of territory near the Afghan border in recent years.

Analysts warn however that Pakistan has yet to tackle the root causes of extremism, and militants retain the ability to carry out attacks.

Last month a drone strike killed the leader of the Pakistani Taliban, Mulla Fazlullah, in neighbouring Afghanistan in what the Pakistani army called a "positive development" that also sparked fears of reprisals.