Mend said on Thursday that it used the Italians to highlight the problems of the Niger Delta to the people of Italy.
"The Italians were originally supposed to have been kept until the exit of the Nigerian despot," Mend said, referring to Olusegun Obasanjo, the country's president, who is due to step down in May.
"Their premature release is in response to pleas from certain quarters, which could not be ignored. This will not be repeated for future captives we will shortly take for as long as the Nigerian government refuses to address our demands."
Mend, which emerged in late 2005 and forced the closure of a fifth of Nigerian output with attacks on oil facilities in February 2006, said in a statement it would step up attacks on the industry and stage bombings across the delta.
"We will take more hostages and concentrate on locations believed to be secure to dispel the false sense of security being felt by some in the oil industry and foreign industry watchers," it said in a statement.
Heavily armed fighters dropped the freed Italians with a small group of journalists at an oil company boat yard on the outskirts of Port Harcourt.
"We were treated very well by the militants. We were in a jungle, they treated us better than they treated themselves," Arena said.
"The only complaint I have is they kept us too long as hostages because if it was short maybe we can understand they are fighting for freedom, but to keep us for 98 days is too long," he said.
Mend wants regional control over the delta's oil resources, the release of two jailed leaders from the area and compensation to delta villages for decades of oil pollution.
Kidnappings of foreign oil workers, mostly for ransoms, multiplied in the delta in January and February although most hostages have now been released.
The only expatriate still in captivity is a French contractor for oil company Total who was abducted by suspected ransom-seekers on February.
Oil companies have reinforced security across the vast wetlands region, and thousands of oil workers and their families have left because of declining security.
Mend has insisted that it does not take ransoms for its hostages, but many groups do.