The military in Nigeria has been accused of killing 1,000 civilians during an upsurge in operations against militants in the oil-rich Niger Delta.
"They...bombard entire communities from the air, sea and land," said the Ijaw National Congress which represents the region's largest ethnic group.
A Nigerian military spokesman dismissed the allegations as unfounded.
A BBC correspondent says the claims are impossible to verify and reflect the people unaccounted for.
The army has freed 17 hostages, mostly foreigners, held by militants fighting for a greater share of oil revenues.
But the military action has "resulted in over a thousand deaths, because we dared to ask for our rights," said Victor Burubo, spokesman for the Ijaw National Congress.
He called on the United Nations to intervene in a BBC interview.
The Movement for the Emancipation of the Niger Delta (MEND) declared "all-out war" on the Nigerian government after an attack on one of its camps deep in the swamp lands on Friday.
A second hostage being held by Mend militants was killed in fighting with government forces.
In the upsurge of violence - the worst in months - the military have used naval gunboats and there are claims of helicopter gunship attacks, the BBC's Caroline Duffield reports from Nigeria.
The army says the offensive is a rescue operation to save the crews of two ships hijacked by gunmen last Thursday, and that nine Filipinos, four Ukrainians and four Nigerians have been freed so far.
A Mend statement said: "We regret to announce that two hostages have been killed by the indiscriminate shelling."
It said their bodies would be given to the Red Cross, but gave no details of nationality.
The group added that Briton Matthew Maguire, who was taken hostage separately last year, was now being moved to a new location.
Mend - the most visible of numerous armed gangs operating in the Delta - says it is fighting for a fairer share of Nigeria's oil wealth for local people.
However they are involved in extortion and massive oil theft as well as kidnapping, our correspondent says.
They had ordered oil companies to pull out their staff by Saturday. The group also says it blew up two major gas pipelines over the weekend.
Analysts expected renewed violence between militants and the military after Mend rejected an amnesty offered by the government.
The government has still not officially said the offer is off the table.