American-born al-Qaeda-linked cleric Anwar al-Awlaki is shown in this 2008 photo in Yemen, where he is now reported to be in hiding. ((Muhammad ud-Deen/Associated Press))

Two civil liberties groups sued the United States government on Monday to try to block its authorization of the killing overseas of U.S.-born cleric Anwar al-Awlaki believed to have inspired recent attacks in the U.S.

The American Civil Liberties Union (ACLU) and the Center for Constitutional Rights filed a lawsuit in U.S. District Court for the District of Columbia for the father of cleric, Nasser al-Awlaki, who's believed to be hiding in his parents' native Yemen. Defendants are U.S. President Barack Obama, CIA Director Leon C. Panetta and Defence Secretary Robert M. Gates.

The groups, both based in New York, said it was unconstitutional to intentionally try to kill al-Awlaki. The Obama administration cited al-Awlaki's growing role with al-Qaeda when it placed him on the CIA's list of targets.

Al-Awlaki was put on the list after U.S. intelligence authorities tied him to Sept. 11 hijackers and concluded he had provided inspiration for those who carried out shootings in Fort Hood, Texas, a failed Times Square car bombing and an attempted Christmas Day bombing of a jetliner approaching Detroit.

The lawsuit seeks a court order declaring that the U.S. Constitution prohibits the government's targeted killings of U.S. citizens, including al-Awlaki, unless there's a concrete and imminent threat to life and there's no other way to prevent it.

In a statement, Justice Department spokesman Matthew Miller defended the U.S. position. He said Congress has authorized the use of all necessary and appropriate force against al-Qaeda and associated groups.

"The U.S. is careful to ensure that all its operations used to prosecute the armed conflict against those forces, including lethal operations, comply with all applicable laws, including the laws of war," Miller said.

He said the U.S. government has the authority under domestic and international law and the responsibility to its citizens to use force to defend itself "in a manner consistent with those laws."

"This administration is using every legal measure available to defeat al-Qaeda, and we will continue to do so as long as its forces pose a threat to this nation," Miller said in the statement.

Al-Awlaki was born in 1971 in New Mexico. His father, who had moved to the U.S. to study agriculture at New Mexico State University in 1966, returned the family in 1978 to Yemen, where he served as agriculture minister.

The younger al-Awlaki returned to the U.S. in 1991 to study civil engineering at Colorado State University before pursuing a master's degree at San Diego State University, followed by doctoral work at George Washington University in Washington, D.C., where he remained until December 2001.

He was a preacher at mosques in California and Virginia before moving to the United Kingdom in 2003 and to Yemen in 2004.

The lawsuit notes al-Awlaki hasn't been publicly indicted for any terrorism-related crime, though Yemeni officials have stated they are taking measures to arrest him. He has been detained by the government of Yemen before and was imprisoned for 18 months there in 2006 and 2007, the lawsuit notes.

Since at least January, al-Awlaki has been hiding in Yemen and has had no communication with his father because to do so would endanger his life, the lawsuit says.

ACLU executive director Anthony D. Romero said a program that authorizes killing U.S. citizens without judicial oversight, due process or disclosed standards is "unconstitutional, unlawful and un-American."

CIA spokesman George Little said his agency acts "in strict accord with American law."