Harare (AFP) - Zimbabwe's 90-year-old President Robert Mugabe fell down a staircase on Wednesday as he walked off a podium after addressing supporters at Harare international airport, an AFP correspondent said.

He had just returned from Ethiopia where he took over the rotating chairmanship of the African Union.

Africa's oldest leader, who turns 91 later this month, had concluded his homecoming speech when he tumbled to his knees on a short flight of stairs in an incident witnessed by journalists and hundreds of supporters.

He was quickly helped up by aides and walked to a waiting car.

Mugabe took over the post of African Union chairman on Friday, replacing Mauritania's President Mohamed Ould Abdel Aziz.

In his speech, Mugabe said he felt honoured that Zimbabwe was elected chair of the AU, pledging to use his position to push for development in the continent.

"The AU must be the engine of development in Africa. We should be the engine of development in Africa," Mugabe said.

"We want real progress to be achieved to develop all our countries."

He said "no African country" opposed Zimbabwe's elevation to lead the organisation.

The former liberation war hero is Africa's third-longest serving leader.

He was met at the airport by a group of cheering supporters who raised placards with messages of support.

"Your greatness resonates across Africa and beyond" read one card.

The leader also used the opportunity to lambast Kenya's President Uhuru Kenyatta for appearing before the International Criminal Court (ICC), when the AU had advised him against it.

"I told him that you will make your father's soul feel ashamed. We don't want to hear that again."

Kenyatta, the son of Kenya's founding president Jomo Kenyatta, was accused of crimes against humanity for his alleged role in the violence that followed the country's disputed presidential election seven years ago.

The charges against him were later withdrawn.