Jordan's Deputy Prime Minister Ayman Safadi said on Sunday that King Abdullah's half-brother and former heir Prince Hamza had liaised with foreign parties over a plot to destabilise the country and had been under investigation for some time.

On Saturday the military said it had issued a warning to the prince over actions targeting "security and stability" in the key US ally. Prince Hamza later said he was under house arrest. Several high-profile figures were also detained.

"The investigations had monitored interferences and communications with foreign parties over the right timing to destabilise Jordan," Safadi said.

These included a foreign intelligence agency contacting Prince Hamza's wife to organise a plane for the couple to leave Jordan, he said.

"Initial investigations showed these activities and movements had reached a stage that directly affected the security and stability of the country, but his majesty decided it was best to talk directly to Prince Hamza, to deal with it within the family to prevent it from being exploited," he said.

The developments are likely to rock Jordan's image as an island of stability in the turbulent Middle East. King Abdullah removed Prince Hamza from his position as heir to the throne in 2004, in a move that consolidated his power.

Although he has been marginalised for years, Prince Hamza has angered the authorities by forging ties with disgruntled figures within powerful tribes.

These people, members of loosely organised opposition groups known as Herak and a vocal opposition based abroad, have in recent weeks called for protests against corruption in a country hard hit by the impact of COVID-19 on the economy, pushing unemployment to record levels and deepening poverty.

Earlier, Hamza's mother Queen Noor, the widow of Jordan's late king, defended her son.

"Praying that truth and justice will prevail for all the innocent victims of this wicked slander," she wrote on Twitter. "God bless and keep them safe."

Safadi said the security services have asked for those involved in the plot be referred to the state security court.

Jordan's neighbours and allies expressed solidarity with King Abdullah over the security measures in the kingdom, an important ally of the United States.

Echoing statements of support by other allies and neighbours of Jordan, Morocco's King Mohammed VI held a phone call with King Abdullah II in which he expressed solidarity and support for the country's security measures, Morocco's royal palace said on Sunday.

Some opposition figures have rallied around Prince Hamza in a move that has displeased the king, officials familiar with the situation said.

But most politicians believe Prince Hamza will be silenced, with little prospect of him posing any threat given that the army and security forces who are backbone of support for the Hashemite dynasty are firmly behind the monarch.

"I think King Abdullah has confirmed himself in the saddle and his son Hussein has consolidated himself as the heir to the throne," said Jawad al Anani, who served as the last royal court chief under the late King Hussein. "This is a page turner event."