Blocked: Zimbabwe bars Jimmy Carter, Kofi Annan and Graça Machel

Former U.S. President Jimmy Carter, former U.N. Secretary General Kofi Annan, and human rights activist Graça Machel were all blocked from entering Zimbabwe this week, after the embattled government of President Robert Mugabe refused to allow them entry for their humanitarian mission.

The government has made it clear they will not cooperate," Annan said at a news conference in Johannesburg after they were blocked. It was the first time that Carter, a Nobel Peace Prize winner, has been denied entry into a country. He said their purpose was only humanitarian and not to be involved in the political conflict that has engulfed the country.

"It seems obvious to me that leaders of the government are immune to reaching out for help for their own people," Carter said at the press conference. The three leaders are all representatives of the Elders, a group formed by former South African President Nelson Mandela, to take on difficult international issues. Machel is Mandela's wife and a longtime human rights activist.

Mugabe, Zimbabwe's 84-year-old president, has led his country since 1980, and is involved in a bitter power dispute with his chief rival, opposition leader Morgan Tsvangirai, who many believe won the disputed presidential election held earlier this year.