Standard & Poor's said Tuesday it has placed its AA long-term credit ratings on Nippon Telegraph and Telephone Corp. and NTT DoCoMo Inc. on CreditWatch, with negative implications.

The A-1 plus short-term ratings on both companies were affirmed, according to the U.S. credit-rating agency.

Placing the companies on CreditWatch reflects the agency's concerns over the impact of a one-time retirement benefit payout scheduled to be carried out under an NTT group restructuring plan, the agency said.

The principal feature of the restructuring plan is its salary reduction program. The retirement scheme will, however, result in "a significant increase" in the near-term debt burden of the NTT group rather than longer-term savings on labor costs, according to S&P.

The agency said that NTT's credit quality will probably be weakened by the plan, remarking that the most likely outcome of the scheme is a lowering of the long-term ratings on both NTT and NTT DoCoMo. It added, however, that these ratings are unlikely to be lowered more than one notch.