Gunmen on a motorcycle have shot and killed a senior member of a leading Pakistani political party.

Zahra Shahid Hussain was shot dead outside her home in the city of Karachi in southern Sindh province.

Ms Hussain was the vice president of former cricket star Imran Khan's Pakistan Tehreek-e-Insaf party (Movement for Justice) in Sindh.

No one has claimed responsibility for the killing.

Mr Khan has accused the Muttahida Quami Movement (MQM) party of ordering the killing.

MQM leaders have denied responsibility and demanded a retraction from Mr Khan.

Mr Khan said: "Her death has sent shockwaves across the rank and file of the party.

"I hold (MQM leader) Altaf Hussain directly responsible for the murder as he openly threatened PTI workers and leaders through public broadcasts.

"I also hold the British government responsible as I had warned them about British citizen Altaf Hussain after his open threats."

"She was an asset of the party. She was a central vice president of this party," said PTI candidate Dr Arif Alvi.

He called for an investigation into the murder, although he expressed little faith in Pakistan's justice system.

"I believe the Sindh government should investigate this murder, look for the killers and get them sentenced, but unfortunately over the last five years nobody has ever been arrested for the killings or tried in a court of law," Dr Alvi said.

Senior police officer Nasir Aftab said that police were also looking into the possibility of murder and robbery.

"Once we further investigate the incident, only then, we will be able to say something more accurate about it," Mr Aftab added.

Mr Khan's party has claimed it was the victim of vote-rigging in several areas of Pakistan, including Karachi, in national elections held on 11 May.

Pakistan's election commission plans to re-run the vote for several national assembly seats, including in Karachi.

A spokesman for Mr Khan's party claimed Ms Hussain was killed to sabotage a re-vote scheduled to be held in Karachi today.