A Sussex zoologist has vanished while following the footsteps of an 18th Century explorer through West Africa.

International law enforcement agency Interpol has been called in to hunt for Christian Velten, who left his Brighton home for an expedition tracing the route of the River Niger in February.

The 28-year-old scientist was due to return last month but has not been in contact with his family since March.

His route, following the continent's third longest river from its source in Mali to its mouth in Nigeria, was to take him through savannah, desert, rainforest and swamps.

The region is inhabited by hippos, giraffes, elephants and leopards. Government travel advice warns of the presence of bandits.

Christian was encouraged to take on the mission by book publishers who believed it would boost his profile as a travel writer.

He was equipped only with a digital camera, a donkey and £3,000.

The last time his parents Pauline and Timothy, who live in Burwash near Heathfield, heard from their only son he was staying in a tent in the grounds of a hotel in the desert village of Kita.

Alex Bird, a childhood friend, said: "Everyone is becoming really worried. Christian was supposed to have returned home on July 22 but he did not reach the airport.

"Normally he emailed fairly regularly but he has not accessed his email account since he has been out there.

"A tracker from Interpol has been employed to follow his trail but very little seems to have been done by the Foreign Office."

Alex said Christian was resourceful but it was out of character for him not to contact his family. He said: "He is quite hard and used to being in dangerous situations.

"As a zoologist he spent a couple of years living in the bush in the Caribbean, charting the flora and fauna with a laptop.

"He wrote a book about his travels and the publishers suggested if he could get his name better known he would stand a better chance of getting published.

"He read about the 18th Century explorer Mungo Park and decided to follow in his footsteps. He had a firm itinerary but now nobody knows where he is."

Mungo Park traced the River Niger for 300 miles and published an acclaimed account of his adventures, Travels In The Interior Districts Of Africa, in 1799.

Six years later he was sent back by the Government to follow the 2,500-mile river to its mouth, but he and his party were attacked in their canoes and Park was drowned.

A spokesman for the Foreign and Commonwealth Office said: "We know Christian is missing and our consul has been in touch with his family but we have not heard anything for a couple of months.

"If anything does arise, we will be in touch with the family straight away."