A group of writers has criticised European and African leaders for not putting Zimbabwe and Darfur at the heart of an upcoming summit.
The US has placed new sanctions on people close to Mr Mugabe
The writers, who include Vaclav Havel and Nadine Gordimer, said the two crises should be on the agenda of next weekend's EU-Africa summit in Portugal.
In an open letter, they called the omission "political cowardice".
Also, the US has announced new sanctions on Zimbabweans close to the country's President, Robert Mugabe.
The US Assistant Secretary of State for African Affairs, Jendayi Frazer, said the travel and financial sanctions were aimed at 40 people who were close to Mr Mugabe.
About six people are facing financial restrictions and US travel bans have been placed on another three dozen people.
"Given Mugabe's escalated use of violence, the United States will be imposing additional sanctions against the worst perpetrators of the regime's brutality," Ms Frazer said.
She said 2007 had been the worst year for human rights abuses in Zimbabwe.
The upcoming EU-Africa summit in Lisbon has been overshadowed by the threat of British Prime Minister Gordon Brown to stay away if Mr Mugabe attends.
Britain, other EU countries and the US accuse Mr Mugabe of economic mismanagement, failure to tackle corruption and human rights violations that have led to soaring inflation and chronic food shortages.
More than 2m people from Darfur have been made homeless
In the Sudanese region of Darfur, four years of conflict have driven more than 2m people from their homes and led to an estimated 200,000 deaths.
The group of more than dozen prominent writers said millions of Europeans and Africans would expect Darfur and Zimbabwe to be at the top of the agenda of the summit.
"What can we say of this political cowardice? We expect our leaders to lead, and lead with moral courage," the writers' said in an open letter published on Tuesday in newspapers in Europe and Africa.
"When they fail to do so they leave all of us morally impoverished."
In addition to Mr Havel and Ms Gordimer, Gunter Grass, Roddy Doyle, Tom Stoppard, Jose Gil, Colm Toibin, Wole Soyinka, Mia Couto, Chimamanda Ngozi Adichie, Gillian Slovo, Ben Okri and JM Coetzee also signed the letter.