Pirates have kidnapped seven crew members during an attack on their ship off the coast of Nigeria.

Six Russians and one Estonian have been abducted from the Liberty 249, operator Bourbon - a French shipping company - confirmed.

Another nine crew members were safe and on their way to the Nigerian port of Onne, a spokeswoman said.

There has been a spate of pirate attacks on cargo ships off the West African coast in recent months.

Bourbon did not disclose the vessel's exact location at the time of the attack on Monday but said an emergency unit had been set up to help the missing sailors.

"The emergency unit has been set up to aim at their rapid liberation under the safest security conditions," the company said in a statement.

The firm has been targeted by pirates in Nigeria before.

In 2010, three French crewmen were kidnapped off a Bourbon-operated ship in an oilfield in the Niger delta, the heart of Africa's biggest oil and gas industry.

The hostages were later released.

The trade in stolen oil has fuelled violence and corruption in the Niger delta.

Although attacks in the region have declined since a 2009 amnesty for militants, piracy is on the rise in parts of West Africa.

In the first six months of this year, the International Maritime Bureau has recorded 17 pirate attacks in Nigerian waters this year - a significant increase on 2011.

Last week, a Greek tanker carrying 32,000 tonnes of petrol went missing with a crew of 24 on its way to the port of Abidjan in the Ivory Coast.

In August, pirates attacked a Greek tanker off the coast of Togo, stealing 3,000 tonnes of fuel.