Moody's downgraded independent explorer Seadrill's credit rating by three notches, citing the continued deterioration of the offshore drilling market.
Independent explorer Enquest's corporate family rating and probability of default rating were also downgraded by one notch, on concerns over its ability to maintain cash flow in the medium-term.
Meanwhile, London-listed firms Tullow Oil and Ithaca were placed on negative watch following a review.
"For the three European E&P companies, cash flow declines in tandem with oil and natural gas prices have weakened credit metrics and put pressure on cash flow generation in 2016/2017, with all three companies executing on relatively large development projects," Moody's said in a note.
"The expansions are being funded by drawing on respective bank facilities, making liquidity and availability of bank funding key credit drivers in the next 12-18 months."
Industry leaders have warned over the effect of low oil prices on the UK North Sea oil and gas industry. One of Britain’s best known oil tycoons, Algy Cluff, said it could collapse without tax breaks, while industry body Oil & Gas UK said its on the "edge of a chasm".
Oil prices have collapsed from a high of over $100 per barrel in June 2014 to around $35 today. This has forced oil companies to slash spending, scale back and even cancel exploration projects and axe jobs.
Moody's expects that the oil market will recover slowly over the next several years. It cut its forecasts for oil prices last month and now expects Brent crude, the global benchmark, to average $33 per barrel in 2016 before rising to $38 a year later.