LAHORE: Grieving families of the Pakistanis who died after a US official opened fire in Lahore have been swept up in a political storm inflamed by hardline Islamists and conspiracy theories.

Raymond Davis, a reported security contractor who Washington says is a diplomat, sparked the latest crisis in Pakistani-US affairs when he shot dead two men carrying guns on motorbikes at a busy traffic junction on January 27.

Washington says he acted in self-defence and President Barack Obama has called for his immediate release on grounds of diplomatic immunity.

A court in Lahore on Thursday put off ruling on whether the American had immunity until March 14.

Police accuse Davis of cold-blooded murder and religious hardliners, already out defending controversial blasphemy laws, have taken to the streets afresh clamouring for Davis to be executed.

The US response to the shootings has been heavily criticised with many Pakistanis saying expressions of regret were too little too late.

Relatives say that neither Pakistani nor US officials came to console them, but Jamaat-ud-Dawa, a charity considered a front for those behind the 2008 Mumbai attacks and blacklisted by the United Nations, have offered support.

“They have extended support to us and they are saying we are with you against America because America has done all wrong,” said a brother of Faizan Haider, one of those killed by Davis, in reference to Jamaat-ud-Dawa and religious party Jamaat-e-Islami.

The brother, Imran Haider, 34, told AFP at his home in a working-class neighbourhood in Lahore that the family had joined political rallies organised by religious groups calling for Davis to stand trial.

The United States has confirmed neither Davis' name nor the precise nature of his assignment.

“We believe Davis is not a diplomat, he's a spy. If he's a diplomat why hasn't the government said that yet? It's fishy,” Imran said.

Media reports have claimed he had a background in the US Special Forces, worked for a security contractor, that he was in contact with the Taliban and even that the two men he shot dead were Pakistani intelligence agents.

In an indication of the level of suspicion in the country, Imran believes that some of the mourners who came to pay their respects and asked whether the family would accept compensation were in fact envoys sent by the Americans.

Lawyers linked to Islamic groups have agreed to represent the families of the two men shot dead, and that of a third man, Ibadur Rehman, who was killed when a US consulate car sent to rescue Davis knocked him down.

Rehman's brother, Aijazur Rehman, said no authorities have offered his family information on who drove the car that killed his brother.

He acknowledged the case was being manipulated by political groups.

“We understand that, but we need some platform to take our voice up front,” he said.

“You can't do anything on your own. They come out for agitation and arrange services. They invite us and we go - it's all parties, not just religious.”

"We want to see the faces of those people who were in that vehicle. We have no enmity with America, we are seeking justice for our brother, nothing else."

One senior US official characterised media speculation about the incident as “unbelievable” but expressed hope that Kerry's visit could help to mend the diplomatic row.

Although some have predicted massive protests and a serious backlash if Davis is released, others have played down any potential fallout.

“They're simply prolonging the agony of the affected families,” said Pakistani analyst Imtiaz Gul of right-wing religious groups.

“They're simply exploiting the emotions of the affected families to turn it into an anti-American campaign, but I don't think the majority of Pakistanis support them.”