Henry Okah was released from detention in the central city of Jos on Monday after being the first senior rebel to accept an amnesty offer from Umaru Yar'Adua, the president.
After Yar'Adua's decision to drop treason and gun-smuggling charges against Okah, Mend said on Monday that it considered "this release as a step towards genuine peace and prosperity if Nigeria is open to frank talks and deals sincerely with the root issues once and for all".
Hours before Okah's release on Monday, Mend fighters killed five oil workers in an attack on an oil tanker wharf in Lagos, the country's most populous city and financial capital, in the first such operation outside the Niger Delta since starting its latest string of attacks.
Mend, which had largely concentrated its campaign on oil facilities and government targets inside the delta region until Monday's attack, called the hit on the depot and loading tankers moored at the facility "unprecedented".
The violence has forced Royal Dutch Shell, Chevron of the US and Italy's Agip to cut around 300,000 barrels per day in production in the last six weeks and has helped support global oil prices.
State-run Nigerian National Petroleum Corporation (NNPC) says that monthly oil revenue this year has dropped to around $1bn from an average of $2.2bn in 2008.
Mend says it is fighting for greater autonomy for the Niger Delta and a fairer distribution of its oil wealth.
Other groups have also said that they would lay down their arms after Okah's release.