As a result of the attacks, Okolobo said Shell had shut some production in the Nembe Creek Trunk pipeline on Monday, but declined to give exact figures.
The Anglo-Dutch oil group recently resumed full deliveries from its offshore Bonga oilfield in Nigeria, after shutting down in June following an attack by Mend.
Bonga lies 120km offshore and has a daily output capacity of 200,000 barrels of oil and 150 million standard cubic feet of gas.
Shell started production at Bonga in November 2005. By May 2007, 100 million barrels of oil had already been exported from there.
The Mend email said: "In keeping with our pledge to resume pipeline attacks within the next 30 days, detonation engineers backed by heavily armed fighters from Mend today ... sabotaged two major pipelines in Rivers state of Nigeria".
"The first pipeline is located in Kula which has been previously sabotaged by us and the second in Rumuekpe, both belonging, we believe, to the Shell Petroleum Development Company."
The latest attack came barely a week after the Mend vowed new attacks to prove that it had not collected money from the Nigerian government.
The head of the Nigerian National Petroleum Corporation, Abubakar Yar'Adua, had told a parliamentary hearing on July 22 that the firm paid $12m in protection fees to Niger delta fighters to enable the repair of a damaged key crude supply pipeline.
"To prove that we are not a part of this deal, the Chanomi creek pipeline and other major pipelines will be destroyed within the next 30 days," Mend said in its email to AFP.
Violent attacks and kidnappings targeting oil companies are a frequent occurrence throughout the Niger Delta.
Some of the armed groups say they are fighting for a greater autonomy and an increased share of oil revenues for the region, while other attacks are carried out by criminal gangs seeking ransoms.