BESANCON, France (AFP) - French authorities arrested an Egyptian national on suspicion of carrying explosives on board a Paris-Venice train but said on Thursday tests showed the bottle posed no major threat.

The 33-year-old, who lives in Italy, was taken off the train late on Wednesday and placed in custody after customs officials carrying out a routine check on the night train at the French city of Dijon confiscated a plastic bottle wrapped in aluminium foil and plaster.

After tests were carried out by a mobile laboratory at a border customs post at Vallorbe in Switzerland, an investigation for "transport and possession of explosive products" has been opened, said Mr Alain Saffar, prosecutor for the nearby city of Besancon.

"The product contained in the bottle had some positive reactions to tests on explosive substances," Mr Saffar said, but cautioned that "these tests are simply an indication, we need more precise analysis to determine exactly what it is".

He added that the arrested Egyptian "said he did not know the nature of the products he was carrying".

A top official for the Doubs region, of which Besancon is the capital, said further testing on the suspicious substances had ruled out all major explosives, chemical and biological threats.

"All major threats have been ruled out," Ms Isabelle Epaillard-Patriat said, listing anthrax, ricin as well as the plague and cholera.

Mr Saffar explained that the customs officials' suspicion was aroused by the fact that the bottle was marked with the word "nitro" - short for the explosive component nitroglycerin.

Eight customs and police agents who came into contact with the suspect and the bottle were being monitored by a medical team, he said.