By Stephanie Nebehay

GENEVA (Reuters) - At least 60,000 migrants have died making their way to new countries over the last 20 years, and their families rarely learned of their fate, the International Organization of Migration (IOM) said on Tuesday.

In a report entitled "Fatal Journeys", the agency called on authorities to ensure the missing are identified and their families traced.

The estimated toll includes a record 5,400 migrants believed to have died in 2015 trying to cross borders, and a further 3,400 who have perished already this year, the IOM said.

Of last year's deaths, 3,770 occurred in the Mediterranean where boats capsized en route to Europe. Others died in the Bay of Bengal and Andaman Sea, the Gulf of Aden and Red Sea and along the U.S.-Mexico border, it said.

"Something in the region of 60,000 migrants across the world have died over the last 20 years," Frank Laczko, director of IOM's global migration data analysis centre, told a Geneva news briefing by telephone from Berlin.

The death rate is particularly high in southeastern Asia, where migrants are trying to reach Thailand and Malaysia.

"The volume of people making those crossings is perhaps lower but the death rate is similar to the Mediterranean death rate," he said, noting it was one in 23 along the central Mediterranean route this year.

However, a separate IOM report said the Central Mediterranean route is probably much more deadly than thought, because "reports of large numbers of bodies being washed up on the shores of North Africa indicate that shipwrecks occur without leaving any traces."

Families are left wondering if their relatives are alive or dead, the IOM said. Without legal proof of death, it may be difficult for spouses to remarry or for families to inherit property.

Fewer than half of the 387 migrants who died when their boats capsized off the Italian island of Lampedusa in October 2013 have been officially identified, it said. In the United States, a cemetery in Arizona contains the remains of at least 800 unidentified individuals believed to be migrants.

"Missing migrants tend to be a low priority for many authorities around the world, because they are often an irregular situation, they may be undocumented and difficult to identify," Laczko said.

Little is known either about the deaths of migrants travelling north overland from sub-Saharan Africa.

At present there is no established common practice for collecting information on migrant deaths between states, or even sometimes between different jurisdictions in the same country, the report said.

"Above all, international and regional databases are needed, in which data that is collected nationally can be stored securely and accessed transnationally," it said.

(Additional reporting by Tom Miles; Editing by John Stonestreet and Hugh Lawson)