Lebanon's outgoing prime minister Saad al-Hariri has made a brief visit to the United Arab Emirates from Saudi Arabia, despite a deepening crisis back home and a rise in regional tensions triggered by his surprise resignation.

Mr Hariri announced his resignation on Saturday during a visit to his ally Saudi Arabia and has not yet returned to Lebanon.

He said he believed there was an assassination plot against him and accused Iran, Saudi Arabia's arch-rival, and its Lebanese ally Hezbollah of sowing strife in the Arab world.

His resignation has thrust Lebanon back into the frontline of the regional rivalry that pits a mostly Sunni bloc led by Saudi Arabia and allied Gulf monarchies against Shia Iran and its allies.

Mr Hariri's office said he had flown to Abu Dhabi and then returned to Riyadh, but it gave no reason for the trip.

It also did not say when he would return home.

Mr Hariri's FutureTV channel said he would also visit Bahrain but gave no reason.

Hezbollah has accused Saudi Arabia of forcing Mr Hariri to resign.

Both Saudi Arabia and aides to the Lebanese leader, whose family made their fortune in the Saudi construction industry, have strongly denied reports that he has been detained or was forced to quit.

Yesterday Saudi accused Lebanon of declaring war against it because of aggression by Hezbollah, dramatically escalating the crisis and threatening to destabilise Lebanon.

Lebanese politicians and Hezbollah have remained silent about the escalation in Saudi rhetoric after a series of consultations with President Michel Aoun, a Hezbollah ally.

A rocket fired from Yemen was intercepted on the outskirts of Riyadh hours after Mr Hariri's resignation on Saturday.

Yesterday Saudi Foreign Minister Adel al-Jubeir told CNN the rocket "was an Iranian missile launched by Hezbollah".

Iranian President Hassan Rouhani has called Mr Aoun to discuss developments, telling him that unity among the Lebanese people would ensure that external strife and regional problems could be overcome, Iran's state news agency IRNA reported.

"Iran will do all it can to bolster the stability of Lebanon," IRNA quoted Mr Rouhani as saying.

Mr Hariri's resignation has triggered the collapse of a national unity government agreed last year in a political deal that united Lebanon's opposing sides.

This had led to Lebanon'sf irst budget since 2005 and to agreement on a new law for a parliamentary election, which could now get derailed.

Mr Aoun has said he will not accept Mr Hariri's resignation until he returns to Lebanon to explain his thinking - a move widely seen as a stalling tactic.

Hezbollah and its allies will struggle to form a government without Mr Hariri or his blessing.

The post of prime minister must be filled by a member of Lebanon's Sunni community, among which he is the most influential politician.