At least 47 people have been killed and 20 others injured after a passenger bus carrying members of the minority Ismaili Shia community was attacked by gunmen on motorcycles in Pakistan's Karachi on Wednesday.

Pakistan Prime Minister Nawaz Sharif has condemned the attack and sought a report from the Interior Ministry. Militant group Pakistan Tehrik-e-Taliban has claimed responsibility of the attack.

Chief of Army Staff (COAS) General Raheel Sharif cancelled his visit to Sri Lanka to rush to Karachi after the attack.

According to eye-witnesses, eight motorcycle-borne gunmen stopped the bus, entered it and opened fire indiscriminately at the passengers. The bus was carrying over 60 passengers of the minority community near Safoora Chowk in Karachi.

The passengers, including men, women and children, were travelling to their place of worship near Aysha Manzil, police said. The gunmen had reportedly used 9 mm pistol to attack the bus, which belonged to Al-Azhar Garden Colony, an Ismaili community housing project in Karachi.

Television channels carried pictures of a pink bus covered in bullet holes and lines of waiting ambulances. The hospital confirmed that at least 25 men and 16 women have been killed in the incident, according to a Dawn.com news report.

Several people are critically injured and death toll is expected to rise. Tehreek-e-Insaf chief Imran Khan took to Twitter to condemn the attack and tweeted, “Stunned & grieved at most condemnable terror attack in Karachi on ordinary citizens of Pak belonging to a most peaceful community.”

Pakistan police and rangers arrived at the scene of the terror attack shortly after the incident and cordoned off the area. A search operation is underway for the gunmen who managed to escape after the incident.

Meanwhile, Prime Minister Narendra Modi took to Twitter to condemn the incident and tweeted. “We stand firmly with the people of Pakistan in this hour of grief. I wish all those injured a quick recovery.”

Pakistan has been witnessing a rise in the sectarian violence in the past years. In the country's deadliest attack against the minority community in last two years, 61 people were killed in a suicide bomb attack in Shikarpur in January earlier this year.

The attack took place when hundreds of people were attending prayers on Friday. In March 2013, a car bomb in Shiite neighbourhood in Karachi killed 45.