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 quoted an official source in the Foreign Ministry as saying.

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"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.

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The Foreign Ministry of Kuwait ordered its citizens to leave Lebanon immediately as well.

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The announcements came the same day that Lebanon said it believes that Saudi Arabia is holding its Prime Minister Saad Hariri and plans to work with foreign states to secure his return to the country, a top Lebanese government official told Reuters on Thursday.

Also on Thursday, French President Emmanuel Macron said he would travel to Saudi Arabia after leaving the United Arab Emirates to see Crown Prince Mohammed bin Salman and discuss crises in Lebanon, Yemen and the wider region.

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"I will go in the coming hours to Saudi Arabia to meet the Crown Prince," he told a news conference in Dubai.

Hariri resigned as prime minister on Saturday in a shock declaration read from Saudi Arabia, pitching Lebanon into deep political crisis and pushing the country back to the forefront of a regional struggle between Saudi Arabia and Iran.

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Lebanese President Michel Aoun is seeking help from diplomats to uncover the mystery surrounding Hariri's resignation, al-Manar TV reported on Thursday.

Hariri reportedly met the French Ambassador to Saudi Arabia Thursday in Riyadh, and has met EU, U.K. and U.S. diplomats in recent days.