Radical Yemeni cleric Anwar al-Awlaqi urged all Muslims serving in the US army to follow the example of Major Nidal Hasan who stands accused of killing 13 comrades, in an Internet video posted on Sunday.

"What Nidal Hasan did was heroic... and I call on all Muslims serving in the US army to follow his path," Awlaqi said in a video posted by Al-Qaeda in the Arabian Peninsula (AQAP) on jihadist websites, the US monitoring group SITE reported.

"Nidal Hasan is one of my students and I am honoured by that," he said in the video, whose authenticity could not immediately be verified.

Hasan, a US army psychiatrist, is accused of opening fire in November on colleagues at Fort Hood, Texas, killing 13 people in what Awlaqi described as "wonderful" action.

Hasan, who is of Palestinian origin, was "defending his nation" and killed American soldiers on their way to Afghanistan and Iraq, the cleric said.

The United States said on Sunday it was actively hunting Awlaqi.

"We are actively trying to find him and many others throughout the world that seek to do our country and to do our interests great harm," White House spokesman Robert Gibbs said.

"He has an agenda just like Al-Qaeda to strike targets in Yemen, throughout the world including here in the United States," Gibbs said on CBS's "Face the Nation" programme.

Awlaqi also defended Umar Farouk Abdulmutallab, the Nigerian student accused of a failed attempt to blow up a Detroit-bound passenger plane with explosives last Christmas.

The operation "was very successful even though it did not even kill anyone," he said.

"Those who were to be killed in the plane are nothing compared to... a million women and children in Palestine, Iraq and Afghanistan," whom he accused the US military of having killed.

"We must treat them the same way and attack them just like they've attacked us."

The US-born cleric in his late 30s said that targeting American civilians is justified in that they "participated in the war because they have elected this (American) government."

US President Barack Obama's administration has authorised the targeted killing of Awlaqi, and Yemen's defence ministry said in April the authorities were tracking him for his alleged links to Al-Qaeda and "terrorist activities."

However, Awlaqi insisted he was safe in Yemen.

"It's not true that I'm being targeted... I am moving around between members of my tribe and in other parts of Yemen," he said, adding that "the Yemeni people hate Americans."

To Al-Qaeda, he said: "You in Al-Qaeda, your speech is that of dignity."

During the 45-minute video, Awlaqi also accused Sanaa of collaborating with Washington to attack Yemenis.

"Yemen is not occupied by the United States. Unfortunately, the situation is worse than that," he said. "The Yemeni government is now telling Americans: you occupy the air and seas and we will... provide the spies on ground who spy on Yemeni Muslims."

However, the preacher said that "Americans cannot launch a third campaign, after Iraq and Afghanistan because if they enter Yemen, their soldiers will be killed in our mountains, valleys, and deserts."

The Abyan region in former South Yemen has become a regrouping area for Islamist militants, including Arab veterans of the 1980s war in Afghanistan against Soviet occupation.

Yemen has intensified operations against the local Al-Qaeda franchise since December, with air strikes killing 34 suspected AQAP members on December 17 in an attack on an alleged training camp in Abyan.

The same number of militants were reportedly killed in another strike on December 24 which targeted a meeting of AQAP militants in Shabwa.

Yemen -- the ancestral homeland of Al-Qaeda leader Osama bin Laden -- has been the scene of several attacks claimed by the group on foreign missions, tourist sites and oil installations.