"Yes it is correct, he (Muqrin) was killed with two other senior militants," a Saudi security source said. He said Muqrin was one of three killed in the al-Malazz area of the capital, Riyadh, in a shootout.

The source said Muqrin and his men were killed while they were trying to dispose of Johnson's body. He said forces had combed four Riyadh areas before honing in on a building where the militants were holed up.

To establish identities, one Saudi official said forensic tests would be conducted on three bodies.

Grief

"Johnson's beheading has gripped the Saudi public in a sense of grief," Saudi journalist Khaled Salih al-Shashri told Aljazeera.

"The killing of al-Muqrin would raise our morale after the gruesome murder of the US victim Johnson."

Regional experts believe killing al-Moqrin, 31, would be a coup for the Saudi government, which has been under intense pressure to halt a wave of attacks against Westerners in the kingdom. 

A second operation was under way in the nearby al-Quds neighborhood, where security forces had surrounded an area in a hunt for suspects, Adel al-Jubeir, diplomatic counsellor to Saudi Crown Prince Abd Allah bin Abd al-Aziz, told reporters in Washington.

US warning

Meanwhile, the US government on Friday warned its citizens in the Gulf region to exercise caution after Johnson's execution in Saudi Arabia. 

"The US government has received information that extremists may be planning to carry out attacks against westerners and oil workers in the Persian Gulf region, beyond Saudi Arabia," the State Department said in a communique. 

"US citizens are reminded to maintain a high level of vigilance
and to take appropriate steps to increase their security awareness."