Oil fell for a second day on June 8 to hit one-month lows after an unexpected surge in U.S. inventories and the return of more Nigerian crude aggravated investor concerns about an already oversupplied market.

The oil price has slipped below $50 a barrel (bbl) despite a pledge by the world's largest exporters to extend an existing output cut of 1.8 million bbl/d into next year in an effort to reduce bulging global inventories.

Adding to concern about supply outstripping demand, Royal Dutch Shell Plc (NYSE: RDS.A) on June 7 lifted force majeure on exports of Nigeria's Forcados crude, bringing all the country's oil grades fully online for the first time in 16 months.

Brent crude was down 26 cents at $47.80/bbl by 6:37 a.m. CT (11:37 GMT), having touched an earlier high of $48.60, while West Texas Intermediate crude futures fell 25 cents to $45.47/bbl.

The market has also come under pressure from news of rising output from Libya, which together with Nigeria is exempt from the production cut made by OPEC and its 11 partners.

"I've been quite bullish for the second half of this year, based on supply and demand balances and I would still not give up on that idea, that rebalancing is going to start in the second half," PVM Oil Associates strategist Tamas Varga said.

"But if Nigerian and Libyan production is picking up as well as they are now, then slowly, I am probably going to have to start changing my mind," Vargas added.

The most actively traded Brent derivatives were bearish sell options that would give the holder the right to sell at $45/bbl and $46/bbl between August and December.

U.S. inventories of crude oil and gasoline surprisingly rose last week as refinery runs declined and exports fell, official data showed on June 7.

"Unless data are released that make the latest inventory build appear an anomaly, oil prices are hardly likely to make any lasting recovery," Commerzbank said in a note.

Sentiment across the broader financial markets also remained jittery.

Former FBI director James Comey's U.S. congressional appearance, a European Central Bank (ECB) policy meeting and the British general election take place on June 8.

Many investors are wary ahead of Comey's Senate appearance as they look for any hints that U.S. President Donald Trump may have been engaged in obstruction of justice—an offense that could lead to impeachment hearings.

ECB policymakers are set to take a more benign view of the economy and will even discuss dropping some of their pledges to ramp up stimulus if needed, sources with direct knowledge of the discussions told Reuters.