Pakistani police have filed a case against a little-known Sindhi rebel group for carrying out a deadly attack on a Chinese-origin dentist couple and their assistant in their clinic in a busy commercial area of this southern port city.

A gunman pretending to be a patient on Wednesday entered the dental clinic of Dr Richard Hu Lee in Saddar area and opened fire on him, his wife Margaret Phen Teyin and assistant Ronald Chow when called inside the cabin for examination.

The assistant died on the spot while the 76-year-old Hu and his wife Margaret, 72, were injured. The attacker escaped after the shooting as his accomplice was waiting nearby with a parked motorbike.

The Karachi Police have registered a case against the outlawed Sindhudesh People's Army, said to be a coalition of Sindhi and Baloch separatist outfits for the attack. The outfit has also claimed responsibility for the attack on social media.

Senior Superintendent (South) Asad Raza said the victims held both Canadian and Pakistani nationalities.

Raja Umer Khattab of the Sindh Counter Terrorism Department said the attack was carried out after a proper reconnaissance since the Chinese-origin dentists have been living in the country for the past 50 years and they have become Pakistani now.I think they were attacked because they were an easy target, he added.

The police believe that the attack was aimed at sabotaging relations between Pakistan and China and it was a conspiracy against uplift projects in the country, he said.

China's Foreign Ministry spokesman Wang Wenbin on Thursday said that the person killed in the attack was not its citizen, refuting reports that the incident could be yet another targeted attack against Chinese nationals in Pakistan.

"We express condolences for the victims of the relevant incident. To my understanding the relevant person is not a Chinese citizen," Wang told a media briefing here when asked whether it was a terrorist attack against Chinese nationals.

Karachi, Pakistan's industrial and financial capital, has seen a series of deadly attacks against the Chinese this year. Most of the attacks have been carried out by Baloch rebel groups seeking an independent Balochistan. However, at least one of the attacks has been carried out by a Sindhi group as well.Chinese nationals who have either been settled in Pakistan for years and set up their own businesses or have come in the wave of officers and workers after the launch of the Chinese Pakistan Economic Corridor projects are seen in big numbers in the Saddar area.

Chinese nationals now settled in Pakistan not only run successful dental clinics but also Chinese restaurants and herbal clinics in Karachi - the capital of the southern Sindh Province which is also home to several Chinese-funded construction projects.

Recent reports said trouble is brewing between the all-weather friends with China increasingly getting critical of Pakistan's failure in protecting Chinese workers who came under periodic attacks from the militant groups.

In April, three Chinese were killed in a suicide blast in Karachi University carried out by the separatist Baloch Liberation Army, which is opposed to China's investments in Balochistan accusing China and Pakistan of exploitation of the resource-rich province.

Some of the other attacks were also attributed to radical Islamic militant outfits in Pakistan.

With recurring attacks, China is reportedly pressing Pakistan to permit the Chinese security agencies to provide security for their personnel which according to press reports, Islamabad is resisting as it meant boots on the ground for Chinese armed forces.