The indexes are key performance benchmarks for international investors in emerging market debt, so membership in them can help a country sell bonds and reduce its borrowing costs.

Sovereign and quasi-sovereign debt issuers from Saudi Arabia, Qatar, the United Arab Emirates, Bahrain and Kuwait will become eligible for the EMBI Global Diversified (EMBIGD), EMBI Global (EMBIG) and EURO-EMBIG indexes, according to the statement, which was seen by Reuters on Wednesday.

Their entry will be phased in between Jan. 31 and Sept. 30. Both conventional bonds and sukuk, or Islamic bonds, will be eligible for inclusion in indexes, but sukuk will need to have a credit rating from at least one of the three major rating agencies to be included.

JP Morgan's decision follows a surge of debt issuance from the Gulf Arab region in the past few years, as low oil prices force most countries to fund part of their state spending in the international debt markets.

Saudi Arabia, Bahrain, Kuwait, Oman and Qatar have issued a quarter of all new debt sold by emerging market countries in each of the last three years.

The inclusion of Gulf Cooperation Council countries in the indexes will leave them representing around 11.2 percent of JP Morgan's EMBI Global Diversified and EMBI Global series, the statement said.

"It is estimated that around $360 billion of assets under management are benchmarked against the EMBIG family, with the EMBIG diversified at around $300 billion," said Zeina Rizk, director of fixed income asset management at Arqaam Capital in Dubai.

Rizk estimated this would translate into about $30 billion (£22.76 billion) of inflows into the five countries' debt.

"Those inflows are not going to come on day one, but the tailwind resulting from the inclusion headline, coupled with pegged currencies, strong oil prices, a relative immunity from trade wars and high credit quality, leads us to the view that the GCC has better value than the rest of emerging markets."

The minimum size for inclusion in the indexes is $500 million, and during the inclusion process, instruments will need to have a maturity date beyond March 2022, the statement said.

(Editing by Andrew Torchia)

By Davide Barbuscia