By Nelson Renteria and Anthony Esposito

SAN SALVADOR (Reuters) - Central American countries are eagerly waiting to see if El Salvador's adoption of bitcoin as parallel legal tender cuts the cost of remittances, an important source of income for millions of people, the region's development bank said.

President Nayib Bukele's congressional allies have already approved legislation giving the cryptocurrency official currency status alongside the U.S. dollar, a first in the world. The move takes effect in September.

Bukele has touted bitcoin adoption as a way to facilitate remittance payments from Salvadorans living abroad.

"Everyone is watching if it goes well for El Salvador and if, for example, the cost of remittances drops substantially ... other countries will probably seek that advantage and adopt it," Dante Mossi, the executive president of the Central American Bank for Economic Integration (CABEI), told Reuters.

Mossi called the plan an "out of this world experiment" geared at increasing financial inclusion in a region where many people lack access to bank accounts or credit cards, and rely on money sent home from relatives living in the United States.

CABEI, the regional development bank, is giving El Salvador technical assistance on implementing the cryptocurrency, an important show of support as the World Bank declined to help, citing environmental and transparency drawbacks.

Mossi said the Central American nations that receive the most remittances are those most likely to favor using bitcoin and underscored that CABEI had a "fiduciary obligation" to support El Salvador in its request for help.

"Guatemala, Honduras and El Salvador are the countries that would have the most to gain if the adoption of bitcoin lowered the cost of sending remittances," said Mossi.

CABEI participated in a recent meeting of the Central American Monetary Council, part of the Central American Integration System (SICA), where participants asked about El Salvador's bitcoin plans and showed interest, he added.

The governments of Guatemala and Honduras did not immediately respond to a request for comment.

Under 1% of the volume of global crossborder remittances are currently in crypto, according to Autonomous Research, but in the future crypto is expected to account for a larger slice of the more than $500 billion in for annual global remittances.

Bitcoin offers, in theory, a quick and cheap way to send money across borders without relying on traditional channels.

CABEI's head of investments Carlos Sanchez said the bank's technical assistance is focused on helping El Salvador design a legal framework for the adoption of bitcoin and to make sure strict international money laundering protocols are adhered to.

The assistance is meant to help El Salvador "navigate waters that have yet to be explored," said Sanchez.

(Reporting by Nelson Renteria and Anthony Esposito; Additional reporting by Orfa Mejia in Tegucigalpa; Editing by Sandra Maler)