RCom, controlled by billionaire Anil Ambani, has traditionally relied on short-term debt and covenant waivers from its banking relationships, but if these waivers are not received it could impact RCom's $300 million bondholders "significantly", Moody's said.
It also said that owing to intense mobile competition, there is no scope for RCom to deleverage.
Meanwhile, India-focused ratings agency ICRA, a subsidiary of Moody's, also downgraded four RCom debt instruments to '[ICRA]D', which signifies instruments either in default or expected to be in default soon.
These include its non-convertible debentures (NCDs) and commercial paper programme.
Another local rating agency, Care Ratings, downgraded the company's NCDs and other debt instruments to default on Tuesday.
RCom sought to reassure investors in its quarterly conference call on Monday, saying it was in talks with lenders to defer loan instalments coming due in the next four months.
The firm plans to repay lenders Rs 11,000 crore ($1.7 billion) and refinance an even larger chunk by end-September, if lenders sign off on the merger of its wireless segment with rival Aircel and sale of a majority stake in its tower unit to Canada's Brookfield.
($1 = Rs 64.65)
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