The U.S. State Department called on American citizens to "exercise increased caution" while travelling to Saudi Arabia, a travel advisory posted on its website said on Wednesday.

It is not just about Saudi Arabia when half of Aramco's production stops. A portion of the consequences of this can be understood from the US president's statement that he has authorized the use of strategic oil reserves.

Saudi Arabia called on southern Yemeni separatists on Thursday to cede control of Aden and voiced its support for the government, an indication that its rift with close ally United Arab Emirates had deepened.

Washington is in talks with Yemen's Iran-aligned Houthi rebels in a bid to end the country's war, a top US official said on Thursday, the first such contact in more than four years.

Yemeni government officials have begun indirect talks with United Arab Emirates-backed southern separatists in the Saudi Red Sea city of Jeddah to end fighting in Aden and other southern provinces, a Yemeni official said on Wednesday.

Saudi Arabia has deployed more troops in southern Yemen to try to contain clashes between nominal allies in the Saudi-led military coalition fighting the Houthis that risk further fragmenting the country.

The United States, Britain and France may be complicit in war crimes in Yemen by arming and providing intelligence and logistics support to a Saudi-led coalition that starves civilians as a war tactic, the United Nations said on Tuesday.

Saudi Arabia is struggling to hold together a military coalition fighting Iran-aligned Houthis in Yemen after local allies turned on each other in a power struggle that has strained Riyadh's alliance with its main regional partner, the United Arab Emirates.

The United Arab Emirates carried out air strikes against government forces in South Yemen on Thursday in support of southern separatists who have deployed reinforcements to take back control of the port of Aden from their supposed allies in the Saudi-led coalition.

Yemeni President Abd-Rabbu Mansour Hadi said on Thursday that government forces had withdrawn from Aden to prevent the city from being destroyed, according to state news agency SABA .

Saudi Arabia and the United Arab Emirates (UAE) have looked inseparable on the world stage, working together to project power in the Middle East and beyond, and courting U.S. President Donald Trump to counter common foe Iran.

Yemen's Houthi movement's air defenses on Wednesday downed a United States drone MQ-9 in Dhamar governorate, southeast of the Houthi-controlled capital Sanaa, the group's Al-Masirah TV quoted the group's military spokesman as saying.

The president of Yemen's Southern Transitional Council (STC) headed with a delegation to the Saudi city of Jeddah on Tuesday after accepting the kingdom's invitation to a summit on the crisis in the Yemeni port city of Aden, the council said.

The refusal of Yemeni southern separatists to hand back control of Aden port has delayed a summit in Saudi Arabia that is due to discuss reshuffling Yemen's ousted government to include the separatists and end the stand-off, three Yemeni sources said.

Saudi Minister of Energy Khalid Al-Falih said on Saturday that Shaybah oilfield was targeted by explosive-laden drones.

Yemen's southern separatists said on Thursday they were ready to attend a summit in Saudi Arabia to resolve a standoff in Aden after they seized the southern port, seat of Yemen's Saudi-backed government.

Yemen's Houthis launched more drone attacks on Saudi Arabia's Abha international airport on Friday, halting air traffic there, the Iran-aligned group's military spokesman said.

A joint Saudi-Emirati military delegation travelled to Aden on Thursday to discuss demands for a pullout of UAE-backed southern separatists from positions they captured in Yemen's interim capital, government and separatist sources said.

Yemen's southern separatists vowed on Wednesday to keep control over Aden, warning the only way out of the impasse that has fractured a Saudi-led military alliance was for Islamists and northerners to be removed from positions of power in the south.

The United Arab Emirates on Monday downplayed a rift with ally Saudi Arabia over Yemen after separatists it backs took control of Aden that was the base of the Saudi-backed government.