Sardar Rahim, a party spokesman said, without directly blaming any individuals or group: "The incident clearly indicates that those who are conspiring against the renewed democracy are behind it."
Sindh province is ruled by a coalition led by the Pakistan People's Party of Benazir Bhutto, the slain former prime minister, and the Muttahida Quami Movement (MQM), a rival to Sharif's party.
The killing of Khan is likely to further pressure Pakistan's new coalition government following its recent dispute over the reinstatement of dozens of judges sacked by Pervez Musharraf, Pakistan's president, last year.
Their eventual reinstatement is likely to cause a problems for Musharraf who considers them hostile to his rule.
But the issue has strained the coalition, with Sharif pulling his PLM-N ministers out of the administration on May 13 after the coalition failed to meet a deadline on how and when to restore the judges.
The main coalition party has added to Musharraf's troubles by drafting a set of constitutional amendments that would reverse many of the changes made to the constitution since Musharraf seized power in a military coup in 1999.
Asif Ali Zardari, who leads the Pakistan People's Party, said his party would present the 62-point draft to the prime minister and send copies to partners in the seven-week-old coalition government.
Musharraf's opponents want to strip him of powers to dissolve parliament, fire the prime minister and appoint the heads of the armed forces.
Elsewhere in Pakistan's northwest a series of separate roadside bomb attacks on Saturday killed three people including a local police chief, according to police officials.