NIAMEY, June 17 (Reuters) - A total of 33 migrants have died in the Sahara desert in Niger while en route to Europe this year, including 18 found dehydrated last week near a road to the border with Algeria, the government of Niger said on Tuesday.

International assessments, however, have put the number closer to 50. Many thousands attempt to cross the vast and inhospitable terrain in order to reach the Libyan coast, where they hope to begin another hazardous trip by boat to Europe.

Six foreigners were found dead near a road between Agadez and the Libyan border on May 12, while nine were found dead on June 2 and four more are missing on a road to Libya, said an interior ministry statement.

"The use of unsecured routes and the refusal to take military convoys is always at the origin of these tragedies," the statement said.

The International Organization for Migration (IOM) said in its own statement on Tuesday that 30 migrants had been found dead in the Sahara near Dirkou in Niger on Monday, bringing to 48 the number of bodies recovered in the country this year.

In a separate case, the bodies of 18 migrants were discovered on Sunday near Arlit, a route to Algeria, IOM said in a statement.

It is likely given reports from migrants and others that far more than 48 migrants have died in Niger's desert this year, said IOM spokesman Joel Millman.

"We know that traffickers are increasing in the area through the desert to Libya. We believe that there has been an undercount (of the dead) because of the remoteness and the difficulty of patrolling," he told Reuters.

Figures offered by Niger's government and IOM differ, but both offer a glimpse into what migration experts say is a hidden tragedy in the Sahara. (Reporting by Abdoulaye Massalaki; Additional reporting and writing by Matthew Mpoke Bigg; Editing by Ken Wills)