A British tourist has been killed in a sectarian attack on a Shia Muslim gathering in Pakistan's largest city of Karachi, The Independent has learned.

Naiyyar Mehdi Zaidi, 60, from London, and two of his brothers were shot and killed when two gunmen on a motorcycle opened fire at a Shia Muslim gathering in the Nazimabad suburb.

Another man and a woman were also killed and six other people injured in the attack.

Mr Zaidi's daughter-in-law Qaisra Khan told The Independent the group was standing outside a Shia ladies prayer meeting when gunmen drove past and tried to gain entry to where the women had gathered.

After guards closed the gates to prevent the gunmen from entering, they reportedly opened fire on the men.

Pakistani security officials inspect the scene of an attack on Shia Muslims in Karachi, Pakistan, 29 October, 2016 (EPA)

Ms Khan said those killed included her father-in-law Mr Zaidi, a UK citizen, Nasir Abbas Zaidi, 45, a US citizen visiting from Seattle, and Baqir Abbas Zaidi, 40, a third brother from Karachi.

Two of Mr Zaidi's other brothers were also severely injured in the attack but Ms Khan has been told they are now in a stable condition.

"Two attackers on a motorbike opened indiscriminate fire on the participants coming for the gathering," senior police official Tayyab Muqaddas Haider told AFP. The attackers fled the scene after the assault, he added.

Ms Khan, who is a Sunni Muslim, said the prayer meeting had been specifically targeted in a sectarian attack.

"I think they're just targeting every minority gathering," she said. "It's indiscriminate. It's a Shia gathering and therefore it was targeted."

Paramilitary soldiers gather at the site attack in Karachi, Pakistan, Saturday, 29 October, 2016 (AP)

The Sunni militant organisation Lashkar-i-Jhangvi al-Alami claimed responsibility for the attack.

“A group of outlawed Lashkar-i-Jhangvi has been active in the city and the threat will persist until their arrest,” Karachi police chief Mushtaq Mahar said.

Mr Zaidi had travelled to Karachi to prepare for another son's wedding next summer. He was due to return to the UK on 7 November.

Ms Khan described her father-in-law, who worked at Habib Bank AG Zurich before retiring around a year ago, as "peaceful, highly intelligent and well loved in the community".

"He always had advice for people and support. He was full of compassion and love for everyone around him. At the moment I know there are people around the world thinking about him."

She said his brothers were similar, adding that "they were a very open minded, tolerant family".

Naiyyar Mehdi Zaidi (left), 60, from London, was killed along with his brother Nasir Abbas Zaidi, 45, from Seattle, USA, in Karachi, Pakistan, on 29 October, 2016 (Qaisra Khan)

She said she hoped reports of the attack would draw attention to sectarian killings. "I often saw these reports and you never think it will happen to you. It's made me realise how destructive these kind of attacks are and how they must stop."

The Foreign and Commonwealth Office said they were not yet aware of the death of Mr Zaidi. FCO advice for tourists visiting Pakistan warns there is a high threat from terrorism, kidnap and sectarian attack throughout the country.