Ethiopian Prime Minister Meles Zenawi, a regional strongman in the volatile Horn of Africa who ruled with an iron fist for over two decades, has died in hospital in Brussels after a long illness, officials said on Tuesday.

Meles, a former rebel who came to power in 1991 after toppling the bloody dictatorship of Mengistu Haile Mariam, set Ethiopia on a path of rapid growth and played a key role in mediating regional conflicts, but also drew criticism for cracking down on opponents and curtailing human rights.

US President Barack Obama led tributes to Meles whom he said deserved "recognition for his lifelong contribution to Ethiopia's development, particularly his unyielding commitment to Ethiopia's poor".

He said Meles had earned his own personal admiration "for his desire to lift millions of Ethiopians out of poverty" through his efforts to improve food security following a meeting at the G-8 in May.

UN Secretary General Ban Ki-moon praised his "exceptional leadership".

The 57-year-old -- a key Western ally in a region home to Al-Qaeda-linked groups -- had not been seen in public since the G20 summit in Mexico in June.

European Commission spokesman Olivier Bailly said Meles had died in Brussels, but the Ethiopian government said only that he had died abroad.

His body arrived back in Addis Ababa on Tuesday evening.

"Prime Minister Meles Zenawi passed away on Monday around midnight," said government spokesman Bereket Simon, adding he had been "struggling to be healthy in the last year."

"He had been recuperating well, but suddenly something happened and he had to be rushed to the ICU (intensive care unit) and they couldn't keep him alive," he added.

Deputy Prime Minister Hailemariam Desalegn, 47, who has also been foreign minister since 2010, will take over interim power, Bereket said.

Unlike many core members of the ruling party, Hailemariam does not hail from the far north of the country but from the Southern Nation, Nationalities and People's Region, the most populous of Ethiopia's nine ethnic regions.

World leaders offered high praise for Meles -- British Prime Minister David Cameron hailed him as "an inspirational spokesman for Africa" -- but rights groups said his death offered a chance to end a brutal crackdown on basic freedoms.

Meles was regularly singled out as one of the continent's worst human rights predators, and Amnesty International called on new leaders to end his government's "ever-increasing repression".

Human Rights Watch called for the next administration to repeal a much-criticised 2009 anti-terrorism law, under which multiple opposition figures and journalists, including two Swedes, have been jailed for lengthy terms.

Ethiopia has declared a state of national mourning, but has not fixed a date for a funeral, said Bereket, adding that "everything is stable" in the country.

Diplomats and analysts in Addis Ababa say it was unclear how the government has been run since Meles was reported to have been hospitalised in June.

Kenyan Prime Minister Raila Odinga told the BBC that Meles had been "a great leader" but said he also "fears for the stability of Ethiopia upon his death."

"The Ethiopian state is very fragile," he said. "I don't know if they have sufficiently prepared for his succession."

Ethiopia faces several internal threats, including the rebel Ogaden National Liberation Front, fighting for greater autonomy in the south-eastern ethnic Somali region. They said they hoped Meles's death "may usher (in) a new era of stability and peace."

Meles was credited with Ethiopia's economic boom in the past decade, with economic growth shooting from 3.8 percent in the 1990s to 10 percent in 2010.

On paper, his government fostered a policy of ethnic federalism, devolving significant powers to regional, ethnically based authorities, but central control remains firmly in the hands of the ruling party.

His death also leaves a major power gap in the Horn of Africa, with Ethiopia playing a key role in the fortunes of many of its neighbours.

Ethiopian troops invaded Somalia for a second time last year -- after a US-backed invasion in 2006 -- and Ethiopia is supporting the fight against Somalia's Al-Qaeda-linked Shebab.

The Shebab said they were celebrating the "uplifting news" Tuesday.

Meles's death could also potentially see changes in the relationship with arch-foe Eritrea, which split from Ethiopia in 1993 before the two spiralled into a bitter 1998-2000 border war in which tens of thousands died.

A peace deal led to a tense standoff, with Meles refusing to pull troops from the border town of Badme, even after an international court ruled the town belonged to Eritrea.

The town has been the source of festering discontent between the two nations ever since. Asmara has so far made no comment on his death.

Meles also played a key role in brokering peace efforts between newly independent South Sudan and its former civil war foe Sudan.