Eritrean Foreign Minister Ali Said died of a heart attack on Sunday, Eritrean Minister of Information Ali Abdu said.
"It's a great loss for this country," he said on Sunday in Asmara, the Eritrean capital.
Abdu said he died in his sleep at 0600 a.m. (0300 GMT). In 1965, Said, the son of a Muslim shepherd, received medical and military training in Syria before going to the field with the Eritrean Liberation Front, one of two groups fighting for independence from Ethiopia.
In the 1970s and 1980s, he became a leading military commander with the other "rebel" group, the Eritrean People's Liberation Front, which later became the People's Front for Democracy and Justice, currently the Red Sea state's governing party.
In 1992, after Eritrea had won its 30-year war for independence from Ethiopia, but had not yet officially declared independence, he was appointed secretary for internal affairs in the provisional government.
In 2000, he was promoted to minister for foreign affairs. Most of his overseas trips were to the Middle East, though he travelled to Russia in April.
"He will be buried on Tuesday at 10 o'clock (0700 GMT)," Abdu said. "Starting from tomorrow, there will be national mourning and flags will be flying at half mast," he said.