The threat came from the Movement for the Emancipation of the Niger Delta, a coalition of armed groups that range from political activists to criminal gangs that's waged an insurgency in the region, the center of Nigeria's oil production, since 2005.
MEND declared it would unleash a campaign of terror against the Islamists led by the group known as Boko Haram -- "Western education is sacrilege" in the Hausa language of the north -- "in defense of Christianity."
Christians have been a major target of Boko Haram and its offshoots and scores of churches have been bombed or burned since the Islamists launched their insurgency in 2009 in the predominantly Muslim north.
As many as 1,700 people, Muslims and Christians, have been killed in the bloodletting triggered by Islamists who increasingly appear to have links with al-Qaida in the Islamic Maghreb, the main jihadist organization in North Africa.
"The bombings of the mosques ... Islamic institutions, large congregations of Islamic events and assassination of clerics that propagate doctrines of hate will form the core mission of this crusade," MEND spokesman Jomo Gbomo said in a statement.
Soon after, at least 185 people were killed in the northeastern fishing community of Baga amid fighting between Islamist extremists and the military. Reports say the Islamists fired rocket-propelled grenades into populated neighborhoods while troops raked the streets with machine gun fire.
The fighting erupted after troops surrounded a mosque where the military believed Boko Haram militants were hiding.
Brig. Gen. Austin Edokpaye, the local military commander, said the militants launched an assault on the army using machine guns and rocket-propelled grenades.
The ferocity of the assault against state forces, combined with the use of relatively heavy weapons not widely seen before, suggested a distinct escalation in the Islamist insurrection.
The West African country's population of 150 million is roughly split evenly between Muslims in the north and Christians in the south.
It's far from clear what kind of a threat MEND, or those groups within it which want to take on Boko Haram, actually poses to the Islamists, or whether Boko Haram is being influenced by al-Qaida.
MEND isn't cohesive but a loose coalition of independent groups with their own agenda and warlords, often more concerned with profit than political or religious objectives.
The communique noted that the threatened MEND offensive would be codenamed Operation Barbarossa, an unfortunate choice.
That was the name of Adolf Hitler's invasion of the Soviet Union in 1941, which launched a chain of events that led to the fall of Berlin four years later.
But if the southern Christian militants, of whom there are around 25,000, do mobilize against the northern Islamists, Nigeria faces a potentially dangerous widening of the current conflict at a critical period in the country's history.
It would sharpen religious and tribal divisions between north and south at a time when Nigeria, Africa's most populous nation, is gripped by political crisis, rampant corruption and serious problems in the oil sector, its economic lifeline.
"We're on the cusp of something imaginable happening," lamented Nigerian journalist Tolu Ogunlesi, who witnessed recent attacks on Muslims in southern Nigeria.
"There's a likelihood that we're going to experience some kind of Christian retaliatory killings for what's happening in the north.
"I'm just not confident that it will be MEND that'll do it. Just like Boko Haram, it isn't a single organization."
Many Nigerians remember how the country was ravaged by civil war from July 6, 1967-Jan. 15, 1970.
The so-called Biafra War was triggered when the southeastern provinces, the self-proclaimed Republic of Biafra, tried to secede from the federal state created after Britain, the colonial power, left.
The conflict, in which as many as 3 million people died, mainly from hunger and disease, was the result of economic, ethnic and religious tensions.
"I don't think this threat should be treated lightly," cautioned Ken Henshaw of the Social Action group in the delta.
"Nigeria's so volatile. Things are getting out of control. Here's a group threatening to kill people, it must be taken seriously."