The number of companies on its B3 Negative and Lower Corporate Ratings List reached a two-year peak as of March 1, Moody's Investors Service says in a new report. And oil and gas companies are accounting for an increasing proportion of the list as falling oil prices lead to more rating downgrades in the energy sector.

''Of the 28 companies that joined the B3 Negative and Lower list in the past three months, 12 were from the oil and gas sector,'' says associate analyst, Julia Chursin. ''While since the list's inception in 2009 companies from the energy sector have represented on average 8% of its total population, at the start of March, 2015 they comprised 14% of the total, the highest percentage ever.''

Moody's B3 Negative and Lower Corporate Ratings List now totals 184 companies, up from 174 a quarter earlier, senior vice president, David Keisman says in ''Oil & Gas Downgrades Help Push List to a Two-Year Peak.'' The latest increase continues a trend of consecutive quarterly upticks that began in the first quarter of 2014, though the list remains far from its record high of 290 companies, set in the first quarter of 2009.

While in the latter half of 2014 most companies left the list via rating upgrades and withdrawals, during the first quarter of 2015 most left it via defaults and rating withdrawals. Among the 18 companies that have dropped off the list this year, 39% filed for bankruptcy protection or completed a distressed exchange, 33% had their ratings withdrawn, and 28% had their ratings or outlook revised upward.

''While previously a majority of companies left the B3 Negative and Lower list via positive rating changes or rating withdrawals, this trend has now reversed, with defaults being the main reason why companies are leaving it,'' Chursin says. ''A continuation of this reversal could signal tough times ahead for speculative-grade issuers.''

Despite slight deterioration in some of Moody's proprietary indicators, overall they do however point to stable credit conditions. The Liquidity-Stress Index reached a six-month high in February due solely to pressure on oil and gas companies, but excluding them, the index fell to a record low. Meanwhile, Moody's Covenant Stress Index, though up slightly in February from the prior month, continues to indicate a low risk of covenant violations among US speculative-grade firms over the coming year.

Moody's new report includes the full list of companies on the B3 Negative and Lower Corporate Ratings List and, where available, the companies' market-implied ratings to help gauge the relative value of the speculative-grade issuers.