The Islamic State terror group has claimed responsibility for the attack on a passenger bus in Karachi on Wednesday by six heavily armed gunmen, who killed at least 43 people and injured 20 others, highlighting Isis' increasing influence in Pakistan.

"Thanks be to Allah, 43 apostates were killed and around 30 were wounded in an attack carried out by Islamic State soldiers on a bus transporting Shia Ismaili infidels in the city of Karachi," the Islamic State said in a statement that was shared by Isis-affiliated Twitter accounts.

Earlier in the day, Jundullah, a splinter group of the Tehreek-e-Taliban, which claimed allegiance to the Islamic State last year, had claimed responsibility for the attack.

Who are Shia Ismailis and Why are They Targeted?

The motorcycle-borne gunmen opened indiscriminate fire on the bus in Karachi's Safora Chowrangi area. About 60 people, including children, of the minority Ismaili Shia community were said to be travelling in the bus.

Among the dead were 16 women, the Dawn reported.

Some news channels also reported that the gunmen entered the bus and then opened fire. The attackers then fled the spot.

Jundullah spokesperson Ahmed Marwat told Reuters that the group was behind the attack.

"These killed people were Ismaili and we consider them kafir (non-Muslim). We had four attackers. In the coming days we will attack Ismailis, Shi'ites and Christians," Marwat told Reuters.

According to some reports, a blood-stained pamphlet was found at the scene in which Isis claimed responsibility.

Last month, an attack on an American aid worker was said to be carried out by Isis supporters, and Islamic State leaflets were found on the scene.

According to The Express Tribune, 60 empty bullet shells of Kalashnikovs were found from the crime scene.

Media reports quoted the police as saying that the Shia Ismaili passengers were the target of the attack as the bus belonged to a housing project of the community -- Al-Azhar Garden Colony -- based in Karachi.

Pakistani and Indian politicians condemned the terror attack.

Stunned & grieved at most condemnable terror attack in Karachi on ordinary citizens of Pak belonging to a most peaceful community. — Imran Khan (@ImranKhanPTI) May 13, 2015

The attack in Karachi is deeply saddening & utterly condemnable. Our thoughts are with the families of the deceased. — Narendra Modi (@narendramodi) May 13, 2015