Image caption Anwar al-Awlaki was seen as an astute communicator and linked to several plots

Relatives of three Americans killed in drone strikes in Yemen are suing top Pentagon and CIA officials, saying the killings were unconstitutional.

Cleric Anwar al-Awlaki and Samir Khan died in September. Awlaki's son Abdulrahman, 16, died in October.

Relatives accuse Defence Secretary Leon Panetta, CIA Director David Petraeus and two military commanders of approving and directing the strikes.

The legality of US use of drones has been in the spotlight in recent weeks.

Awlaki, a radical Islamist cleric born in the state of New Mexico, was a key figure in the Yemen-based group al-Qaeda in the Arabian Peninsula (AQAP). His 16-year-old son was born in Colorado.

Samir Khan was a naturalised US citizen who was involved with Inspire, al-Qaeda's English-language magazine.

The lawsuit was filed by Nasser al-Awlaki, the father of Anwar al-Awlaki, alongside Sarah Khan, mother of Samir Khan.

'Rights violated'

Military commanders Adm William McRaven - head of US special operations - and Lt Gen Joseph Votel were also named in the lawsuit, which is seeking unspecified damages.

There is something terribly wrong when a 16-year-old American boy can be killed by his own government Pardiss Kebriaei, Center for Constitutional Rights How legal are drone strikes?

"The killings violated fundamental rights afforded to all US citizens, including the right not to be deprived of life without due process of law," the legal complaint says.

The Center for Constitutional Rights and the American Civil Liberties Union say they are giving legal assistance to the plaintiffs.

"There is something terribly wrong when a 16-year-old American boy can be killed by his own government without any accountability or explanation," said Pardiss Kebriaei, a lawyer at the Center for Constitutional Rights.

A justice department spokesman told Reuters news agency the department had seen the complaint and was reviewing it.

Correspondents say the case could face procedural hurdles before it is settled.

In March, US Attorney General Eric Holder defended the drone strike policy, saying the US "may use force abroad against a senior operational leader of a foreign terrorist organisation with which the United States is at war - even if that individual happens to be a US citizen."

The US government has not officially accepted responsibility for carrying out the strikes.

The Department of Justice, which is expected to represent the defendants, could contend that state secrets would be exposed as part of the evidence needed to argue the case.

Before the lethal strike took place, Awlaki's father had tried to get a court injunction to stop US attempts to kill his son. The request was dismissed on the grounds that Mr Awlaki could not sue in place of his son.