(AFP) - Jordan said Sunday it had foiled a plot against the kingdom involving a half-brother of King Abdullah II, arresting at least 16 suspects it accused of "sedition" and alleging foreign complicity.

Hamzah bin Hussein -- a former crown prince stripped of that title by Abdullah in 2004 -- and the others had worked with foreign parties to "undermine the security" of Jordan, Deputy Prime Minister Ayman Safadi said.

Washington, Gulf allies and the Arab League were quick to stress their support for Abdullah's pro-Western government, seen as an anchor of stability in the Middle East.

Egyptian President Abdel Fattah al-Sisi called the monarch to affirm Cairo's "full solidarity" with Jordan and express "complete support of the recent decisions (he) has taken to ensure the stability and security of the kingdom", according to a presidency statement.

Hamzah, 41, had Saturday released a video message via the BBC in which he accused Jordan's rulers of nepotism and corruption and charged that he had been placed under house arrest.

He lashed out at Jordan's "ruling system", saying several of his friends had been arrested, his security detail removed and his internet and phone lines cut.

He denied being part of "any conspiracy or nefarious organisation", but said the country of 10 million people had "become stymied in corruption, in nepotism and in misrule" and that no one was allowed to criticise the authorities.

State-run news agency Petra said among those arrested for "security reasons" were former close aide to the royal family Bassem Awadallah, chief of the royal court in 2007-08, and Sherif Hassan bin Zaid.

Safadi, who is also foreign minister, said another 14 to 16 suspects had been arrested.

Security services monitored "contacts with foreign parties aiming to destabilise Jordan's security," including an alleged offer to spirit Hamzah's wife out of the country, he said.

Safadi declined to identify the alleged foreign parties or say what the charges were, but said authorities acted because the alleged conspirators were "talking about timing".

"This sedition was nipped in the bud," Safadi said.

- US closely watching -

Hamzah's mother, American-born Queen Noor, defended her son and the others, tweeting that she was "praying that truth and justice will prevail for all the innocent victims of this wicked slander."

Hamzah is the eldest son of Queen Noor and the late king Hussein.

Abdullah had appointed Hamzah, a popular figure close to tribal leaders, crown prince in 1999 in line with Hussein's dying wishes, but stripped him of the title in 2004 and gave it to his eldest son Hussein.

A Jordanian analyst, who declined to be named, said Hamzah had recently "stepped up his criticism of what he described as corruption within the government in front of his circle of friends.

"There is certainly resentment on his part, because he has never digested losing his title of crown prince."

Jordan's Joint Chiefs of Staff head Major General Yousef Huneiti said Hamzah had not been detained, but "asked to stop some activities that could be used to shake the stability and security of Jordan".

The Washington Post had first reported that the former crown prince was "placed under restriction" as part of a probe into an alleged plot to unseat the king.

It "included at least one other Jordanian royal as well as tribal leaders and members of the country's security establishment."

- Upcoming centenary -

Parliament speaker Abdul Monem Odat said "Jordan has firmly and resolutely stopped any attempt to undermine its security and stability and sent a clear message to those who King Abdullah II describes as the opponents of Jordanian policies".

The US State Department said Washington was "closely following" the events, stressing that key partner Abdullah "has our full support".

Saudi Arabia's King Salman and Crown Prince Mohammed bin Salman both phoned Jordan's king to express their support, Saudi state media said.

Israel's Defense Minister Benny Gantz said "Jordan is a neighbour and strategic ally with whom we have peaceful relations. We need to do everything necessary to maintain that alliance."

Awadallah, a US-educated former finance and planning minister, was close to the king but has also been a controversial figure in Jordan. Before becoming royal court chief in 2007, he was head of the king's cabinet.

He played a key role pushing for economic reforms before he resigned in 2008 amid criticism over alleged interference in sensitive political and economic issues.

Saturday's security sweep came as Jordan prepares to mark 100 years since the new kingdom, then named Transjordan, was established alongside Palestine under British mandate. It declared independence in 1946.

But Barah Mikail, an academic at Saint Louis University Madrid and director of consulting firm Stractegia, said the country "would come off well in the face of a plot hatched against it, particularly as the kingdom's anniversary approaches".

Jordan "is very well structured in terms of intelligence and national networks, and tight control, in particular of individuals suspected of being able to harm the monarchy, is a fact," he said.

© Agence France-Presse