"The immigration department has sent letters to all the embassies and foreign missions abroad not to issue visas for ordinary passport holders from Nigeria, Zimbabwe, Swaziland and Ghana," Yadav Khanal, the Nepal foreign ministry spokesman, told AFP on Wednesday.

He said the decision was taken by officials at the home ministry because of "security reasons." He did not elaborate.

Home Ministry spokesman Baman Prasad Neupane said that the new regulation took effect on January 1, 2007.

Police officials, who declined to be named, said the government took the step because several nationals from the four countries had been arrested recently in connection with drug and currency smuggling operations in Nepal.