The step-grandmother of former US president Barack Obama died at a hospital in Kenya, her relatives said on Monday.

Sarah Obama was believed to be at 99-years old. The reason for her death was not disclosed.

"It is true she has gone to be with the Lord, she passed on this morning," said an emotional Marsat Onyango, her daughter.

Sarah Obama, popularly known as Mama Sarah, had been admitted to the Jaramogi Oginga Odinga Teaching and Referral Hospital in Kisumu.

Family spokesman Sheikh Musa Ismail said she had been unwell for a week but had tested negative for Covid-19.

He said the former president had "sent his condolences."

She was the second wife of President Obama’s grandfather Hussein Onyango Obama, a herbalist and a village elder who fought for the British in Burma, now called Myanmar. She helped raise the former president's father.

The family is part of Kenya's Luo ethnic group.

She will be remembered as a philanthropist who promoted education for girls and orphans and even raised some orphans in her home.

The Mama Sarah Obama Foundation helped provide food and education to children who lost their parents.

President Obama often showed affection toward her and referred to her as “Granny” in his memoir, “Dreams from My Father.” He described meeting her during his 1988 trip to his father’s homeland and their initial awkwardness as they struggled to communicate which developed into a warm bond.

She attended his first inauguration as president in 2009. Later, Obama spoke about his grandmother again in his September 2014 speech to the United Nations General Assembly.

After leaving office, he travelled to her home in the village of Kogelo in 2018, joking he had been unable to do so in 2015 as his jet was too big to land in Kisumu.

Obama is linked to his Kenyan family via his father Barack senior, a pipe-smoking economist who Obama has admitted he "never truly" knew.

Details on preparations for her burial will be announced later, said her daughter.

Sarah Obama was Muslim and it’s not clear whether she will be buried according to her faith’s practices that dictate she should be buried within 24 hours of death.