Heavily armed Baloch gunmen killed at least 37 people in separate attacks in Pakistan's restive Balochistan province on Monday as insurgent attacks spiked in the region bordering Afghanistan.

According to government and security officials, on the intervening night of Sunday and Monday, militants belonging to outlawed separatist groups carried out four attacks in which 37 people were killed.

The Inter-Services Public Relations, the media wing of the Pakistan Army, said in a statement that 21 terrorists were also killed in cleanup operations launched after the attacks.

Earlier, Baloch gunmen killed at least 37 people in two separate attacks in Balochistan province on Monday.

In the first incident, at least 23 people from Pakistan’s Punjab province were killed in a targeted attack in Balochistan’s Musakhel district after gunmen offloaded them from buses and checked their identities.

According to Musakhail assistant commissioner Najeeb Kakar, around 10 heavily armed men blocked the inter-provincial highway in Rarasham and offloaded passengers from several buses.

“The dead are reportedly from Punjab,” he said. Some of the vehicles were also set on fire.

Musakhel is approximately 450km northeast of Quetta, the capital of Balochistan.

The rebels regularly target people from the Punjab province by alleging that Punjabis are dominant in the armed forces, which have been fighting the militants in the province.

Senior superintendent of police (SSP) Ayub Khoso said: “The passengers were told to get off the buses and shot dead after being identified from their national ID cards,” Khoso said.

“Most of those killed belonged to southern Punjab and some are from Khyber Pakhtunkhwa suggesting they were killed because of their ethnic background,” he added.

In another incident, officials said 11 people were killed in Kalat, also in Balochistan. The deceased include five civilians and six security personnel, authorities said.

Kalat is 150km to the south of Quetta and is predominantly inhabited by ethnic Baloch tribes.

The banned militant organisation, Baloch Liberation Army, claimed responsibility for these attacks, which coincided with the 18th death anniversary of ethnic Baloch tribal leader Nawab Akbar Khan Bugti who was killed in a military operation.

The militant groups co-named its violence "Operation Heroof" and simultaneously launched a slew of attacks in various districts of the province.

“They carried out the attack early in the morning and then escaped into the nearby mountains,” SSP Kalat, Dostain Dashti, said on telephone.

Four other security personnel were killed in another attack in Kolpur area of Bolan district while in the Kadkucha area in Mastung district, militants also attacked a Levies post compound and left a body behind.

The militants also blew up a bridge on a main railway track in Bolan.

Balochistan province has been the centre of clashes between separatist groups and security personnel for a while now and militants have frequently carried out attacks targeting workers, labourers or pilgrims passing through or working in Balochistan.

These groups have also frequently targeted security and government personnel and installations in different parts of the province.