"My name is Paul Marshal Johnson and I am a citizen of the United States," a slurred voice with an American accent said in a recording on an Islamist website on Tuesday. "I work for Apache helicopters."

The threat to kill Johnson, which follows a spate of suicide bombings and shootings in the past six weeks, raised the stakes in al-Qaeda's war to topple the kingdom's pro-US monarchy and drive out Westerners from the world's largest oil exporter.

"If the tyrants in the Saudi government want to secure the release of the American hostage, they must release our mujahideen held hostage in its jails. They have 72 hours from today or else we will sacrifice him," said an al-Qaeda statement carried on Sawt al Jihad Web site.

The statement, dated Tuesday, was signed by the Qaeda Organisation in the Arabian Peninsula. Sawt al Jihad has carried similar messages in the past.

On Saturday, al-Qaeda said it was holding Johnson, a US engineer and the first Westerner to be kidnapped in a wave of militant attacks in the kingdom that began more than a year ago. It also claimed responsibility for killing American Kenneth Scroggs, who was shot dead as he parked his car at his villa.

Saudi government foreign affairs adviser Adel al-Jubeir told CNN that his government was looking into the situation and consulting the United States on what actions to take.

"And then we will make decisions on what the next steps are but our history has been one of not negotiating with terrorists," he said.

De facto Saudi ruler Crown Prince Abdullah said on Wednesday the kingdom would strike against al-Qaeda soon.

"We have forces and until now they have not appeared but you will see them in the coming days," he said in remarks on Saudi media. "We can only be patient for so long and from now on you will see things that will reassure you."

Concerns about security in Saudi Arabia had helped push world oil prices to record highs recently before oil producers said they would increase output.

A US State Department official said Washington will use every appropriate resource to gain Johnson's safe release in co-operation with the Saudi government.

"Saudi Arabian authorities can count on the United States to give them an assistance they request or need ... The US government makes no concessions to individuals or groups holding our citizens," the State Department official said.

The leader of al-Qaeda in Saudi Arabia, Abdul Aziz al-Muqrin, promised in earlier statements that 2004 would be "bloody and miserable" for the kingdom.