Dieghi was among three Italian and one Lebanese man seized in a December 7 raid on an oil export terminal operated by Agip, a subsidiary of Italian oil firm Eni SpA.
Eni confirmed Dieghi's release in a brief statement posted on its website, and said he was in good health and undergoing medical checkups at Eni's base in the Nigerian oil hub of Port Harcourt.
Working on release
The company said it was continuing to work with Nigerian authorities and Italy's foreign ministry towards the release of the three other workers still held captive.
However, the fighters said no discussions were under way.
"There are no discussions ongoing about the release of the remaining two Italians and one Lebanese still in our custody," the Mend statement said. "They are being held indefinitely."
Nigerian government officials and the Chinese foreign ministry said the five Chinese telecommunications workers kidnapped on January 5 had been freed on Wednesday night.
No group had claimed responsibility for their capture and the Movement for the Emancipation of the Niger Delta had denied it was involved.
Asked if a ransom had been paid to ensure the workers' release, Liu Jianchao, the Chinese foreign ministry spokesman, said in Beijing: "I'm sorry. I can't give any details on the specific process."
Emeke Woke, a Rivers state government official, said the Chinese were handed over to their company's lawyers in that state.
Teleken, based in the southwestern Chinese city of Chengdu, had been contracted to install equipment to upgrade rural telephone service in Nigeria.
The company is a subsidiary of China Telecom Corp, the country's biggest telephone company.