The UAE’s President Sheikh Khalifa bin Zayed Al-Nahyan died Friday aged 73 after a years-lengthy battle with illness, triggering a length of mourning and a handover of power inside the oil-wealthy Gulf state.

Sheikh Khalifa, who was in office during a time of rocketing fortunes for the United Arab Emirates but was rarely seen in public, is likely to be replaced by his half-brother, Abu Dhabi Crown Prince Mohammed bin Zayed, who was already viewed as the country’s de facto ruler.

The most visible testament to Sheikh Khalifa is the world’s tallest building, Dubai’s Burj Khalifa, which was renamed after he bailed out the debt-hit emirate when the global financial crisis struck in 2009.

“The Ministry of Presidential Affairs has mourned to the UAE people, Arab and Islamic nations and the world the death of President His Highness Sheikh Khalifa bin Zayed Al-Nahyan,” the official WAM news agency tweeted.

The ministry announced 40 days of mourning, with flags at half-mast from Friday and work suspended in the public and private sectors for the first three days.

Funeral prayers were set to take place later on Friday.

As condolences poured in from abroad, US President Joe Biden called Sheikh Khalifa “a true partner and friend of the United States”.

Lebanon, Kuwait and neighbouring Oman also ordered three days of mourning.

Sheikh Khalifa took over as the UAE’s second president in November 2004, succeeding his father as the 16th ruler of Abu Dhabi, the richest of the federation’s seven emirates.

He has made few public appearances since 2014, when he had surgery following a stroke, although he has continued to issue rulings.

The cause of death was not immediately released.

The UAE, a former British protectorate that was founded in 1971, has gone from a desert outpost to a booming state in its short history, fuelled by its oil wealth and Dubai’s rise as a trading and financial centre.

The Arab world’s second-biggest economy behind Saudi Arabia is now wielding growing political influence, filling a space ceded by traditional powers such as Egypt, Iraq and Syria.

The country of some 10 million also joined military campaigns in Libya and Yemen and broke ranks with much of the Arab world to establish ties with Israel in 2020.

The bearded Sheikh Khalifa had cut a frail figure on his occasional public appearances, while his half-brother hosted world leaders and led diplomatic forays abroad.

“The Emirates has lost its virtuous son and leader of the ‘stage of empowerment’ and the trustee of its blessed journey,” Mohammed bin Zayed tweeted.

“His stances, achievements, wisdom, generosity and initiatives are in every corner of the nation… Khalifa bin Zayed, my brother… may God have mercy on you and grant you access to paradise.”

Sheikh Khalifa, who had no formal higher education, led the UAE as Dubai emerged as a tourism and trade hub while Abu Dhabi pumped oil as a key OPEC player.

He came to the rescue of Dubai when it was hit by the global financial crisis, extending a multi-billion-dollar lifeline to the emirate.

Dubai’s ruler, UAE vice president Sheikh Mohammed bin Rashid Al-Maktoum, said the country was mourning “with hearts filled with sadness”.

Israeli Prime Minister Naftali Bennett and Qatari Emir Sheikh Tamim bin Hamad Al-Thani also supplied their condolences, along with effective neighbours Saudi Arabia and India, Egypt, Syria, Pakistan, and Ethiopia, among others.