Saudi Arabia has warned its citizens against travelling to Lebanon and asked those who are in the country to leave as soon as possible, the kingdom's official news agency has said, quoting an official source in the Foreign Ministry.

"Due to the circumstances in the Lebanese Republic, the kingdom asks its citizens who are visiting or residing" in the country to leave it as soon as possible, the source added.

It comes after two senior Lebanese government officials said they believe Saad al-Hariri, who resigned as prime minister on Saturday while in Saudi Arabia, is being held by Riyadh.

The two officials said Lebanon plans to work with foreign states to secure his return.

A third source, a senior politician close to Saudi-allied Mr Hariri, said Saudi Arabia had ordered him to resign and put him under house arrest.

A fourth source familiar with the situation said Saudi Arabia was controlling and limiting his movement.

Mr Hariri's shock resignation, read out on television from Saudi Arabia, pitched Lebanon into a deep political crisis and pushed the country back to the forefront of a regional struggle between Saudi Arabia and Iran.

In his resignation speech, Mr Hariri denounced Iran and the Lebanese Shia group Hezbollah for sowing strife in Arab states and said he feared assassination.

His father, a veteran former prime minister, was killed by a bomb in 2005.

A surprise even to his aides, Mr Hariri's exit fuelled wide speculation that the Sunni Muslim politician, long an ally of Saudi Arabia, was coerced into stepping as Saudi seeks to hit back against Iran and its Lebanese ally Hezbollah.

Saudi Arabia and members of Mr Hariri's Future Movement have denied reports that he is under house arrest.

He has put out no statements himself to that effect, and has not denied that his movements are being restricted.

Hemade a one-day flying visit to the United Arab Emirates, a close Saudi ally, earlier this week before returning to Saudi Arabia.

"Keeping Hariri with restricted freedom in Riyadh is an attack on Lebanese sovereignty. Our dignity is his dignity. We will work with (foreign) states to return him to Beirut," said the senior Lebanese official, speaking on condition of anonymity because the government had yet to declare this position.

Saudi Arabia says Mr Hariri resigned because Hezbollah, which was included in Mr Hariri's coalition government, had "hijacked" Lebanon's political system.

Mr Hariri's office said in statement he had received the French ambassador to Saudi Arabia and had also met the head of the EU mission to Saudi Arabia yesterday, as well as the British ambassador and the US charge d'affaires.

Mr Hariri came to office last year in a political deal that made the Hezbollah-allied Christian politician Michel Aoun head of state and produced a coalition government grouping most Lebanese parties including Hezbollah.

Saudi Arabia blessed the government at the time, but has been fiercely critical of the Hariri-led government since he stepped down, saying it failed to act against Hezbollah, whose guerrilla army is far more powerful than the weak state.

Saudi Arabia had wanted Mr Hariri to take a tougher stance towards Hezbollah, and he failed to do so, the fourth source said.

"He was functioning as if it is business as usual, so the Saudis had to accelerate the process and to force a resignation."