President Donald Trump said on Friday that the United States was "locked and loaded" to respond if Iran killed protesters, prompting Tehran to warn that intervention would destabilise the region.

Protesters and security forces clashed in several Iranian cities on Thursday, with six people reported killed, the first deaths since the unrest escalated.

Shopkeepers in the capital Tehran went on strike on Sunday over high prices and economic stagnation, actions that have since spread into a protest movement that has swept into other parts of the country.

DUBAI, Jan 2 (Reuters) - U.S. President Donald Trump threatened on Friday to come to the aid of protesters in Iran if security forces fire on them, days into unrest that has left several dead and posed the biggest internal threat to Iranian authorities in years.

"We are locked and loaded and ready to go," he said in a social media post. The United States struck Iranian nuclear facilities in June, joining an Israeli air campaign that targeted Tehran's atomic programme and military leadership.

Yemen's UAE-backed separatists announced a two-year transition to independence Friday despite reporting 20 deaths in airstrikes from a Saudi-led coalition trying to roll back their weeks-long offensive across the country's south.

A separatist military official and medical sources reported 20 fighters dead in air raids on two military bases as the coalition also targeted an airport and other sites.

Jan 2 (Reuters) - U.S. President Donald Trump on Friday said that if Iran shoots and violently kills peaceful protesters, the United States of America will come to their rescue.

"We are locked and loaded and ready to go," he said in a Truth Social post.

This follows the deaths of several people as Iran's biggest protests in three years over economic hardship turned violent across multiple provinces.

DUBAI, Jan 2 (Reuters) - Saudi Arabia's ambassador to Yemen said on Friday that Aidarus Al-Zubaidi, the leader of the UAE-backed Southern Transitional Council (STC), refused landing permission the previous day for a plane carrying a Saudi delegation to Aden.

The halt in flights at Aden international airport was the latest sign of a deepening crisis between Gulf powers Saudi Arabia and the United Arab Emirates, whose rivalry is reshaping war-torn Yemen.

For years, the United Arab Emirates and Saudi Arabia projected geopolitical and economic power across the Gulf and beyond, seemingly in tandem.

But a growing rivalry and struggle for influence has come to a head -- most recently in Yemen -- following years of divergence over a tangle of competing interests that reach from regional waterways to the corridors of power in Washington, analysts say.

DUBAI, Jan 1 (Reuters) - Air traffic at Yemen's Aden international airport was halted on Thursday as tensions persisted between Saudi Arabia and the United Arab Emirates, two Gulf powers whose rivalry is reshaping war-torn Yemen.

The Saudi-backed, internationally recognized Yemeni government ordered new restrictions on flights to and from the UAE, aiming to curb escalating tensions in Yemen, a Saudi source told Reuters.

Protesters and security forces clashed in several Iranian cities on Thursday, with six reported killed, the first deaths since the cost-of-living demonstrations broke out.

The protests began on Sunday in Tehran, where shopkeepers went on strike over high prices and economic stagnation, and have since spread to other parts of the country.

On Thursday, Iran's Fars news agency reported two people killed in clashes between security forces and protesters in the city of Lordegan, in the province of Chaharmahal and Bakhtiari, and three in Azna, in neighbouring Lorestan province.

By Elwely Elwelly

DUBAI, Jan 1 (Reuters) - Several people were killed during unrest in Iran on Thursday, an Iranian news agency and a rights group reported, as the biggest protests to hit the country for three years over soaring inflation sparked violence in several provinces.

The semi-official Fars news agency cited a "source with knowledge" as saying several people were killed in clashes on Thursday morning between police and what it said were armed protesters in Lordegan in western Iran.

Separatists from Yemen's Southern Transitional Council said on Thursday that Saudi-aligned government forces would enter territories seized by the UAE-backed group, in a step that appeared unlikely to satisfy Riyadh after it repeatedly demanded their full withdrawal.

A surprise offensive by the STC, in which the separatists took control of resource-rich Hadramawt and Mahra provinces last month, has brought the United Arab Emirates and Saudi Arabia, officially allies in Yemen, to a point of unprecedented tension.