Nigerian President Goodluck Jonathan's office has hit back at ex-leader Olusegun Obasanjo for a "provocative" letter
Pirates have freed three Italians kidnapped off the coast of Nigeria last month, says Italy's foreign ministry.
Gunmen stormed the MV Asso Ventuno some 40 nautical miles (74km) off Bayelsa state in Nigeria's oil-producing Niger Delta region on 23 December.
The ministry gave no details of a fourth sailor, thought to be Ukrainian, who was kidnapped with the Italians.
Kidnapping is a lucrative enterprise worth millions of dollars a year in Nigeria, Africa's main oil producer.
Most people in the Niger Delta are extremely poor.
Oil workers and other foreign nationals are often targeted because companies pay high ransom money to secure their employees' release, correspondents say.
Italy's foreign ministry would not comment on whether a ransom was paid over the Asso Ventuno kidnappings.
It said the released sailors would be flown back to Italy later on Wednesday.
One of those released, Emiliano Astarita, told his father by telephone the kidnap victims had been treated well in captivity, Italy's Ansa news agency reported.
After the kidnapping, remaining crew-members - operated by Augusta Offshore SpA, a Naples-based shipping company in southern Italy - had sailed the vessel to safety.