Lebanon's prime minister Saad Hariri, who resigned over the weekend while in Saudi Arabia, has left Riyadh for the United Arab Emirates where is he is to meet a senior official, his office said.

Much is unknown about Mr Hariri's unexpected resignation, which stunned Lebanon, threw its government into disarray and prompted a frenzy of speculation.

One of the rumours is that he has been under house arrest in Saudi Arabia, or in some way forced to do the Saudis' bidding.

His departure for the UAE could appear to dispel that, except that the Gulf federation is a close Saudi ally - the two countries have been spearheading a war in Yemen since 2015 against the country's Shiite rebels who are backed by Iran.

Saudi Arabia on Tuesday sent its harshest threat to the Lebanese government, dominated by the Shiite militant group Hezbollah, accusing it of declaring war against the kingdom through support Hezbollah allegedly provides to Yemen's rebels.

Saudi minister for gulf affairs Thamer al-Sabhan said the situation was "very sensitive and worrying" and that Saudi patience over Hezbollah was running thin.

"The Party of Satan has become a tool of killing and terrorism for the kingdom," he told the Saudi-owned Al-Arabiya TV, using a subversion of Hezbollah's name, which in Arabic means the Party of God.

Hezbollah and the Houthis, as Yemen's Shiite rebels are known, have denied any role by the Lebanese group in the war in the Arabian Peninsula country.

"The Party of Satan is represented in the Lebanese government and the government should be aware of such dangers," Mr al-Sabhan said.

"The Lebanese government will be dealt with as a government that has declared war against the kingdom of Saudi Arabia."

He added that the Lebanese people should be aware of the dangers "before we reach a point of no return".

Mr Hariri's office said he would meet the UAE's powerful crown prince, Sheikh Mohammed bin Zayed Al Nahyan, in Abu Dhabi.