The G7 group has called for unimpeded access for aid workers to Ethiopia's northern Tigray region, where the UN says some 350,000 people are living in famine conditions.
The world's seven largest so-called advanced economies also demanded an immediate ceasefire and the withdrawal of Eritrean troops.
The Eritreans are fighting alongside government forces against local rebels.
Ethiopia denies there is a famine and says the Eritreans have begun to leave.
Meeting in the UK, the G7 group, which includes the US, EU, and the UK, also said they would donate a billion Covid-19 vaccines to poorer countries by next year and raise $100bn (£70bn) to help them cut carbon emissions.
Hundreds of people traveled to Falmouth, near to where the G7 summit is being held, to protest the situation in Tigray.
The war, which broke out in Ethiopia last November, has killed thousands of people and forced some 1.7 million from their homes.
"We are deeply concerned by the ongoing conflict... and reports of an unfolding major humanitarian tragedy, including potentially hundreds of thousands in famine conditions," said a statement issued after the G7 summit.
Ethiopian and Eritrean forces have been accused of deliberately blocking aid supplies to Tigray - a charge Ethiopia denies, saying it is distributing assistance as it restores order to the region.
Aid workers report that some of the supplies which do get through are stolen.
