Saudi Arabia has ordered all of its citizens out of Lebanon as soon as possible and warned against travel to the country, the kingdom's state-run Saudi Press Agency says.

Key points: Saad Hariri left Lebanon for a trip to Gulf Arab region but hasn't returned

Saad Hariri left Lebanon for a trip to Gulf Arab region but hasn't returned Lebanese government officials say Riyadh is holding Mr Hariri captive

Lebanese government officials say Riyadh is holding Mr Hariri captive Saudi Arabia warned earlier this week that it would "deal with" Lebanon

"Due to the circumstances in the Lebanese Republic, the kingdom asks its citizens who are visiting or residing there to leave immediately," a Foreign Ministry source quoted by the news agency said, adding that Saudis were advised not to travel to Lebanon from any country.

Kuwait, Bahrain and the United Arab Emirates have similarly warned their citizens against travel to Lebanon.

Lebanese Prime Minister Saad Hariri resigned on Saturday while in Saudi Arabia, accusing Iran and the Lebanese Shiite group Hezbollah of sowing strife in Arab states and saying he feared assassination.

Top Lebanese government officials have accused Riyadh of holding Mr Hariri captive.

Mr Hariri's Future Movement party demanded on Thursday that he return from the kingdom immediately.

"The return of the Lebanese Prime Minister, the national leader, Saad Hariri, and the head of the Future Movement, is necessary to restore the dignity and respect to Lebanon at home and abroad," said a former premier, Fouad Saniora, in a statement read on TV.

Mr Saniora heads the party's bloc in Parliament.

Saudi Arabia's Foreign Minister Adel al-Jubeir warned earlier this week his Government would "deal with" Lebanon as a hostile state as long as the militant group Hezbollah was in the Lebanese Government.

He said Hezbollah's participation in government is an "act of war" against Saudi Arabia.

Saudi Arabia sees Hezbollah as a proxy of Iran amid a spiralling rivalry between the two regional Sunni and Shiite heavyweights.

The tensions come as Saudi Arabia detained 201 princes and businessmen as part of a $100 billion probe into corruption in the the kingdom.

Resignation remains shrouded in mystery

Mr Hariri left Lebanon last week for a trip to Saudi Arabia and the Gulf Arab region and shocked the country by announcing his resignation.

Saad Hariri announced his resignation as Prime Minister on the weekend. ( AP: Hassan Ammar, file )

Even close associates in his party have been uncertain why he resigned or when he would return.

Mr Hariri is known to have left the kingdom just once since announcing his resignation, on a brief trip to the United Arab Emirates, before returning to Riyadh.

His resignation remains shrouded in mystery.

In his televised statement, he said Lebanon had been taken hostage by Hezbollah — even though he formed a coalition government with the group, which has a substantial representation in Parliament, less than one year ago.

Hezbollah's leader and one of Lebanon's most powerful figures, Hassan Nasrallah, has speculated openly that Mr Hariri was being held against his will in the kingdom and even said that it appeared as if Saudi Arabia forced the resignation.

The Future Movement statement suggested the party was moving toward the same conclusion.

Reuters/AP