A lawyer representing civilian victims of drone strikes in Pakistan has accused the US government of blocking his appearance at a conference in Washington this month by failing to grant him a visa.

Shahzad Akbar, who founded the Islamabad-based human rights organisation Foundation for Fundamental Rights, says he has failed to secure a visa since he began suing the CIA over the killing of Pakistani civilians by US drones. The case is expected to be heard this month in Islamabad.

Sponsors of the drone summit Killing and Spying by Remote Control, including the Center for Constitutional Rights, Reprieve and the peace group Code Pink, have criticised the failure to grant a visa to Akbar, who they say provides a much-needed voice for the victims of drone strikes in tribal Pakistan.

Speaking from Pakistan by telephone, Akbar said: "Denying a visa to people like me is denying Americans their right to know what the US government and its intelligence community are doing to children, women and other civilians in this part of the world. The CIA, which operated the drones in Pakistan, does not want anyone challenging their killing spree. But the American people should have a right to know."

He was due to be a key speaker at the Washington conference on 28-29 April. It aims to "inform the American public about the widespread and rapidly expanding deployment of both lethal and surveillance drones, including drone use in the United States" and promised participants the opportunity to listen to the personal stories of Pakistani drone-strike victims, according to its website.

It is the second time that Akbar, who has been granted US visas in the past, has tried and failed to enter the US to speak at an event addressing human rights concerns over the use of drones.

Last year, he was invited to participate at a conference at Columbia University law school in New York. His US visa had expired three months earlier, but his application in May was effectively put on hold.

Despite renewed inquiries to the US ambassador in Islamabad this year, he has not had an explanation for the delay of over a year. His last US visa, issued in 2009, was processed in three working days, he said.

Although Akbar has travelled to the US in the past and has worked for US companies, he has not been granted permission to return since he began speaking out against drone attacks in his homeland in 2010.

Organisers of the drone summit described the failure of the US to grant him a visa as "outrageous".

Cortney Busch, an investigator at Reprieve, said: "If the Obama administration continues to avoid discussion of its drones programme, Shahzad Akbar must be allowed to tell the stories of the numerous victims he has met in order to give voice to a silenced community. Shahzad remains one of only a handful of people worldwide able to shed light on these clandestine CIA attacks. The US must allow honest debate on a policy which is killing hundreds of civilians and straining relations between the US and Pakistan."

Akbar's relationship with the US "changed dramatically", he said, when he took on the case of Karim Khan, a journalist from North Waziristan. Khan's son, 18, a government employee, and brother, 35, a schoolteacher and father of a toddler, were killed when two missiles fired from what he believes was a CIA-operated drone struck his home in 2009.

In November, Akbar initiated legal notices against the CIA and the US secretary of defence for their deaths. The legal case accuses, among others, Jonathan Banks, the CIA station chief in Islamabad, as being responsible for the deaths. Banks left the country in 2010 after the legal action blew his cover.

The subject of drone strikes is shrouded in secrecy because they are operated by the CIA. The Pakistani government criticises them in public, but documents published by Wikileaks in 2008 revealed they are privately supported by officials.

In February, President Barack Obama said drone strikes are "kept on a very tight leash" and "have not caused a huge number of civilian casualties". However, a growing body of evidence, including that provided by Akbar, tells a different story.

A three-month investigation by the Bureau of Investigative Journalism, published days after Obama's comments, found that between 282 and 535 civilians, including 60 minors, have been credibly reported as killed as a result of drone strikes since Obama took office three years ago. It also found that at least 50 civilians were killed in follow-up strikes when they had gone to help victims.

The subject of drone strikes is shrouded in secrecy because they are operated by the CIA. Photograph: Kirsty Wigglesworth/AP

Akbar said that US drones are so prevalent in the tribal area of Waziristan, which borders Afghanistan, that there are four or five hovering in the air at any one time. The children call them bangana, because of the noise they make.

According to Akbar, 95% of drone strikes in Pakistan happen in Waziristan, a sparsely populated area where many people work in transportation.

With his first-hand knowledge of the victims of the attacks, Akbar provides a challenge to the US narrative of "precision strikes against high value targets".

"The US is saying that there are almost no civilian deaths. I'm challenging that. I have 80 people that are all civilians. It can be proved that they are not terrorists or assisting terrorists. This is in spite of the hardships in getting into Waziristan."

He said that by blocking his trip to Washington, the US authorities were preventing him revealing the true impact of the drones strategy.

"I wanted to tell American people about the human stories behind these strikes," he said. "These are the people who will choose the next American president and what they decide will have far-reaching implications around the world.

"They think the American war on terror is making America safe, but it is not making America safe. It is creating enemies. A huge number of people being killed in these strikes are civilians. These people have no justice, no system where they can file a claim if a child is killed. They are not terrorists. But you have terrorist in the area who tries to recruit people. These are the things people need to know. They need to push their lawmakers to stop this drone policy."

He plans to file public interest litigation, the equivalent of a judicial review, at the Peshawar high court next week on behalf of 80 families. Victims of the strikes include women and children, the youngest being seven, and one person in a wheelchair.

Akbar is challenging both the CIA and the Pakistani government over the use of drones.

"We are basically asking to clarify what Pakistan's role is. They say one thing in public and, we are learning from Wikileaks, another in private," he said. "They have a duty to protect their citizen. We want to push the Pakistan government to take this matter to the international criminal court or to the security council."