Nigerian coastal and fishing communities were on Thursday put on alert after Shell admitted to an oil spill that is likely to be the worst in the area for a decade, according to government officials..

An oil spill on the shores of the Niger Delta swamps. Shell has said the recent oil spill is likely to be worst in a decade. Photograph: Pius Utomi Ekpei/AFP/Getty Images The company said up to 40,000 barrels of crude oil was spilled on Wednesday while it was transferred from a floating oil platform to a tanker 75 miles off the coast of the Niger delta.

All production from the Bonga field, which produces around 200,000 barrels a day, was last night suspended. "Early indications show that less than 40,000 barrels of oil have leaked in total. Spill response procedures have been initiated and emergency control and spill risk procedures are up and running," said Tony Okonedo, a Shell Nigeria spokesman.

Satellite pictures obtained by independent monitors Skytruth suggested that the spill was 70km-long and was spread over 923 square kilometers (356 sq miles).

But a leading Nigerian human rights group said Shell's figures about the quantity of oil spilled or the clean-up could not be relied on. "Shell says 40,000 barrels were spilled and production was shut but we do not trust them because past incidents show that the company consistently under-reports the amounts and impacts of its carelessness," said Nnimmo Bassey, head of Environmental Rights Action, based in Lagos.

"We are alerting fisher folks and coastal communities to be on the look out. It just adds to the list of Shell's environmental atrocities in the Niger delta."

The spill, one of the worst off the coast of Nigeria in 10 years, is particularly embarrassing for Shell, coming only four months after a major UN study said it could take Shell and other oil companies 30 years and $1bn to clean spills in Ogoniland, one small part of the oil-rich delta. The company also admitted responsibility in August for two major spills in the Bodo region of the delta that took place in 2008, but has yet to pay compensation.

Shell, which works in partnership with the Nigerian government in the delta, claims that 98% of all its oil spills are caused by vandalism, theft or sabotage by militants and only a minimal amount by deteriorating infrastructure. But this is disputed by communities.

Yesterday Shell said it had also closed a Gulf of Mexico deep drilling operation after spilling 319 barrels of contaminated fluids.