Trump -- who in his first term sent tensions spiraling by suggesting that Egypt may bomb the project -- offered his services as he hailed Egyptian President Abdel Fattah al-Sisi, an ally who helped broker a ceasefire in Gaza.

"I am ready to restart US mediation between Egypt and Ethiopia to responsibly resolve the question of 'The Nile Water Sharing' once and for all," Trump said a letter to Sisi posted on social media by the White House.

"The United States affirms that no state in this region should unilaterally control the precious resources of the Nile, and disadvantage its neighbors in the process," Trump said.

He promised that the Grand Ethiopian Renaissance Dam (GERD) would be "at the very top of my agenda."

He said he hoped for a formula to provide a predictable water supply for Egypt and Sudan and let Ethiopia either sell or give electricity to the two downstream countries.

Prime Minister Abiy Ahmed in September inaugurated the GERD, Africa's largest hydropower project built at a cost of $4 billion, which will more than double Ethiopia's current electricity capacity.

Sisi has called the dam an existential threat, with parched Egypt counting on the Nile for 97 percent of its water needs.

Mediation efforts by the United States, World Bank, Russia, United Arab Emirates and the African Union have all faltered over the past decade in resolving the disputes over the dam, a unifying symbol of pride in Ethiopia.

Trump, speaking near the end of his first term, had also said he hoped for a resolution and said Egypt's response if not, "They'll blow up that dam."

The remarks stunned both Egypt and Ethiopia, which summoned the US ambassador for clarification and vowed to resist any attack.

Trump has a warm relationship with Sisi, a former general who took power in a 2013 coup and has since severely repressed the opposition.

Trump was once quoted as calling Sisi "my favorite dictator."