Puerto Rico's public transit system will shut down on Monday if lawmakers do not increase a tax on oil, the U.S. commonwealth's Government Development Bank said, in what would be a major escalation of the island's debt crisis.

The possible shutdown of Puerto Rico's buses and commuter train services threatens a fragile economy and would be a further blow to residents already facing budget cuts.

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"The HTA (Highways and Transportation Authority) does not have sufficient income or liquidity to meet its payroll obligations for December," the bank said in a statement. "Given this fiscal picture, as indicated by the governor, public transport services will close next Monday."