Shahzad Akbar says visa hold-up means he cannot take his clients to Capitol Hill to testify on CIA drone strikes in Pakistan

The US government is being accused of derailing a congressional hearing that would be the first to hear testimony from survivors of an alleged CIA drone strike by failing to grant the family's lawyer a visa.

Shahzad Akbar, a legal fellow with the British human rights group Reprieve and the director of the Pakistan-based Foundation for Fundamental Rights, says the state department is preventing him from taking his clients to Capitol Hill next week. The hearing would mark the first time US lawmakers heard directly from drone strike survivors.

Akbar's clients, Rafiq ur-Rehman, his 13-year-old son, Zubair, and his nine-year-old daughter, Nabila, are from the tribal regions of north Waziristan. The children were injured in the alleged US strike on the village of Tappi last year. Their grandmother – Rehman's mother, Mamana – was killed.

Rehman and his children have spent months making preparations to visit Washington after being invited by US representatives to testify in the ad hoc hearing on drone strikes.

According to Akbar, his clients' visas for the trip have been approved, but his has not. He believes the hold-up is political.

"It's not like my name is scratched because there is some sort of confusion. My name is blocked," Akbar told the Guardian. "Before I started drone investigations I never had an issue with US visa. In fact, I had a US diplomatic visa for two years."

This is the third tangle Akbar has experienced with US authorities over a visa since 2011, a year after he began investigating drone strikes. In April, Akbar said he was being prevented from speaking at a human rights conference in Washington because of a delay processing his application. He was eventually granted entry.

Florida congressman Alan Grayson, who helped spearhead the effort to bring the Rehman family to the US, told the Guardian that the state department had not given "a specific reason as to why [Akbar]'s having trouble getting in".

"I don't know why the State Department has taken this action, but I think it's extremely important that when it comes to a national security matter like drone attacks, we hear not only from the proponents of these attacks, but also from the victims," Grayson said.

"We have a chronic problem in Congress that when the administration is involved in one side of the issue, we rarely hear about the other side of the issue.

"This is true with regard to NSA domestic spying. This is true with regard to proposed military intervention in Syria. And it's also true with regard to the drone attacks in Pakistan and in Yemen."

He added: "I think Congress and the American people simply need to hear both sides of the story, and that's why we invited these witnesses to come and testify."

Akbar is an internationally-known critic of US drone strikes in Pakistan, representing over 150 survivors of alleged US strikes and their family members in litigation against CIA and government officials in Pakistan.

His most recent request for visa approval began last month. Documents reviewed by the Guardian show he submitted his non-immigrant state department visa application on 26 August, while Rehman and his children submitted theirs on 28 August.

Akbar says he and his clients' visa interviews were booked through the American Express offices in Islamabad and held in the US embassy there; his on 4 September, his clients' on 6 September.

Akbar said his interview got off to an atypical start when an American official escorted him to a separate room for questioning. "Normally when you go to the embassy, there are different counters in the big hall and everyone is interviewed at the counter, and this is where the victims – Rafiq and his children – were interviewed, but I was interviewed in a separate room," Akbar said.

"They got the result within a week and I'm still waiting for my visa."

Akbar said the woman who interviewed him told him he had been "flagged."

"She said they know me very well, so they don't need really to clarify anything. They were aware that I was coming. They were aware of the invitation from the congressman," Akbar said.

He claimed the woman told him her job was to identify immigration or flight risks, neither of which he was, then said that because his "history" with the US, "my visa has been flagged."

A State Department spokeswoman said "two agents" were reviewing questions concerning Akbar's visa submitted by the Guardian but did not respond with answers before publication.

"I keep checking and they still tell me that it's in administrative process," Akbar said. "They say they cannot tell me how long it will take." A state department information sheet indicates the total wait time for a non-immigrant visa in Islamabad, including the appointment interview and processing, should not exceed 13 days. Akbar began the process one month ago.

Akbar believes another government agency may be blocking his visit. "We brought litigation, civil litigation and civil charges, against CIA officials in Pakistan for their role in drone strikes. I think it's pretty clear that I have been blacklisted because of that."

The Rehman family had been invited to Congress to describe the afternoon of 24 October, when their village was hit by four missiles, allegedly fired by drones that had been buzzing overhead for days.

Nabila was playing outside when the munitions struck. She tried to run but was burned by the blast. She and Zubair were hospitalized for injuries they sustained. Zubair required surgery to remove the shrapnel from his leg.

Their father, who was finishing work when the attack happened, returned home to find a smoking crater, bleeding children and dead cattle. Scattered in a field a considerable distance from the blast site were the remains of his 67-year-old mother.

Initial reports citing unnamed security officials claimed as many as four "militants" were killed in the attack. North Waziristan is well-known for its militant population and has been a consistent target of the CIA's drone campaign.

But Rehman says his mother – the wife of a retired headmaster – was the only person killed in the strike, and maintains there were no fighters present when the missiles were fired. Akbar said he was able to make contact with Rehman a week after the strike and, as a result, managed to collect a substantial body of evidence indicating it was unlawful.

"There is no evidence of any militant killed," Akbar said. He said the only people injured were children; a total of nine, three seriously. Neither US nor Pakistani officials have disclosed the names or any other details of the militants they claim were the targets.

Akbar said Rehman and his children agreed to travel to the US on the condition he would join them as their lawyer, and they are now considering abandoning the trip. "This was a big plunge for these people," he said.

Robert Greenwald, a US filmmaker, was introduced to the Rehman family through Akbar while working on an forthcoming documentary, Unmanned, examining US drone strikes. On Tuesday, Greenwald's organization, Brave New Films, released a one-minute video featuring Rehman and his children, that called on the state department to allow Akbar to accompany his clients on their trip to the US.

"It's very, very upsetting that the efforts of the state department may really stop something that's pure democracy," Greenwald told the Guardian.