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May 21 (Reuters) - Moody's Investors Service on Thursday cut Puerto Rico's bond ratings further into junk status, saying the troubled U.S. territory would not be able to access markets by the June 30 fiscal year-end and might have to prioritize debt payments.

Cash resources at the Puerto Rico Government Development Bank may be fully depleted by the end of August in the absence of market access or emergency actions to preserve cash, Moody's said.

"We believe that the commonwealth will not be able to complete its planned financing (which was to replenish cash at the GDB) before the end of the fiscal year," Moody's said in a statement.

The rating actions affect about $54.8 billion.

Puerto Rico is struggling with $72 billion in debt and faces a bond payment of $655.2 million on its general obligation bonds on July 1, according to data from the GDB.

Moody's said Puerto Rico would have to take "cash-conservation measures" and might defer principal payments on GDB notes and other debt further down the capital structure.

Moody's downgraded Puerto Rico's general obligation and guaranteed bonds to Caa2 from Caa1 and cut the rating on the GDB's notes to Ca from Caa1.

The Caa2 rating falls in the "extremely speculative" category, while Ca indicates default or little prospect for recovery.

The outlook for all affected securities remains negative, Moody's said.

(Reporting by Sai Sachin R in Bengaluru and Edward Krudy in New York; Editing by Lisa Von Ahn)