(CNN) -- A Nigerian militant group released pictures Sunday of two Britons identified as captive oil workers, saying the men were "alive and well" and that more such Western workers would be taken hostage if the country does not stop exporting its oil wealth.
The Movement for the Emancipation of the Niger Delta released this image of two men it claims are British hostages.
The photos, sent in an e-mail by the Movement for the Emancipation of the Niger Delta (MEND), show the men, identified as Robin Barry Hughes and Matthew John Maguire, standing on a dirt path wearing dirty shorts and flip flops.
The pictures were "recent," MEND said in a written statement, but it is unclear when they were taken.
MEND, formed in 2005, has taken American and European oil workers hostages in the past. The group is calling for more of the African nation's oil wealth to be pumped into the region -- instead of going to foreign investors -- and the release of political prisoners.
The United States Agency for International Development says more than 70 percent of Nigeria lives on less than a dollar a day -- the population is among the 20 poorest in the world.
Nigeria's federal government and oil companies split oil profits roughly 60-40. The money is then supposed to make its way down to the local governments to fund various projects, but little money actually reaches its intended destination.
The country's anti-corruption agency estimates between $300 billion to $400 billion has been stolen or wasted over the last 50 years.
"Our policy on kidnapping high value oil workers from Western Europe and North America remains unchanged and will continue to form an integral part of our pressure strategy in the emancipation struggle in 2009," MEND said in its statement. Watch special correspondent Lisa Ling meet militant group in a secret location
A spokeswoman for the British Foreign Office said the government was aware of the pictures.
"We call for their immediate and unconditional release and will remain in close contact with their families," the spokeswoman said, though she declined to elaborate on whether the families had seen the photos. "Our thoughts are with them on this deeply distressful time."
Violence in oil-rich Nigeria has been limiting crude supplies out of the country. MEND has been attacking oil pipelines in retaliation against government forces, limiting the amount of crude oil that can be exported.
MEND also repeated its threat that the men would be held hostage until the Nigerian government releases one of the group's members, Henry Okah, who was taken into custody last year and, according to local reports, is charged with treason.