YAOUNDE/PARIS (Reuters) - Gunmen have kidnapped a French priest working in northern Cameroon, authorities said on Thursday, nine months after Nigerian Islamist sect Boko Haram seized a French family in the same lawless area.

A Roman Catholic Church official in France said around 15 gunmen stormed into the parish church of Nguetchewe, 10 km (6 miles) from the Nigerian border, demanding money late on Wednesday.

The 42-year-old priest, Georges Vandenbeusch, had time to alert the French embassy by phone before the gunmen burst into his room. They marched him barefoot across the village before fleeing on motorbikes.

"His suitcase was found on the road to Nigeria with only a chequebook in it," Monsigor Gerard Daucourt, the bishop in Paris responsible for the priest, told a news conference.

A regional Cameroonian official said he might have been targeted by Boko Haram for sheltering Nigerians who had fled across the border to escape attacks by the group, which has struck churches, schools and security posts.

"This was premeditated, targeting a Catholic priest who was French because we are in Mali and in Mali it's considered that we are soiling Muslim lands," said former intelligence officer Louis Caprioli, who now advises for private security firm GEOS.

Whatever the motive, the kidnapping was the latest in a series of attacks on French targets in West Africa since France launched a military intervention in Mali in January to oust al Qaeda Islamists there, who had forged links with Boko Haram.

Nigeria has complained that the Far North region of Cameroon is being used by Boko Haram militants to transport weapons and hide from a six-month military offensive against them. It has appealed to Cameroon to tighten border security.

The United States formally designated Boko Haram and the Nigerian Islamist militant group Ansaru as foreign terrorist organizations on Wednesday, making it a crime to provide them with material support.

Augustine Fonka Awa, governor of Far North region, told Reuters he had gone to Nguetchewe with security forces to investigate the kidnapping and expressed fears the priest had been taken to Nigeria.

"We suspect that he was being blamed by Boko Haram Islamist rebels for hosting some Nigerian people who escaped the attacks in their country," Awa said.

HOLLANDE CONCERNED

An official at the Paris prosecutor's office said an investigation had been opened into "kidnapping and illegal confinement by a group linked to a terrorist organization".

France's Foreign Ministry said it was trying to establish the identity of the kidnappers. Paris considers the region high risk for kidnapping and has warned its citizens to leave, but Vandenbeusch had insisted on completing his mission.

"All must be done and all will be done so that he can be freed as quickly as possible," French President Francois Hollande said during a visit to Monaco.

Vandenbeusch arrived in Cameroon in 2011, having previously been a priest in the Paris suburb of Sceaux.

Alain Marsaud, a French lawmaker representing voters based overseas, told France Info radio that Vandenbeusch had been meeting nuns and other people when armed men took him.

"We have good reason to believe that it may have been people from Nigeria and Boko Haram in particular," he said.

Henri Djionyang, the vicar general of Maroua in Cameroon, told Radio France International that locals said the kidnappers spoke to each other in English. The Far North province of Cameroon is Francophone, while English is spoken in Nigeria.

If the work of Boko Haram, the attack would be the latest by Islamists on French in the region.

Two French journalists were killed in Mali this month and responsibility was claimed by al Qaeda in the Islamic Maghreb in retaliation for France's operations in Mali. Security experts say the killings might have resulted from a botched kidnapping.

Boko Haram kidnapped a French family of seven on holiday in northern Cameroon in February and released them in April.

Hollande denied a ransom had been paid but a confidential Nigerian government report seen by Reuters said Boko Haram was given the equivalent of $3.15 million by French and Cameroonian negotiators.

Four French hostages abducted in northern Niger were also freed last month. French media reported a 20 million-euro ransom had been paid, something Paris strongly denied.

(Additional reporting by Brian Love and Gerard Bon in Paris and Aziz El-Yaakoubi in Rabat; Writing by Daniel Flynn and Bate Felix; Editing by Matthew Mpoke Bigg and Andrew Roche)