Crude oil prices slipped lower on Monday as a UN Security Council vote highlighted the prospect of Iranian oil returning to the global market.

West Texas Intermediate for delivery in August, the US benchmark, fell for the fourth straight session, dropping 74 cents to $50.15 a barrel on the New York Mercantile Exchange.

London, global benchmark Brent North Sea crude for September delivery closed at $56.65 a barrel, down 45 cents from Friday's settlement.

For the WTI contract, Monday's closing level was the lowest since March 17, when it settled at $43.46. Several times during the session it dipped below $50, flirting with its lowest close since April 2.

"The market is in search of a bottom," said Gene McGillian of Tradition Energy.

According to the analyst, "with the dollar rising the market is still trying to price in the idea we could have additional supplies to oil come to the market" in the near future, following the expected lifting of economic sanctions against Iran.

Earlier in the day the United Nations Security Council unanimously adopted a resolution endorsing the historic deal reached last week between six major powers and Iran on Tehran's nuclear program.

Citi Futures analyst Tim Evans said that oil was swept up Monday in a wider decline in commodity prices, pointing out gold dropped to new five-year lows.

"We continue to view the current physical surplus as sufficient to drive price lower, but more Iranian production will certainly add to the overhang and push an eventual rebalancing of market further down the road," Evans said.

The Islamic Republic's exports could reach a potential 2.4 million barrels per day in 2016, from 1.6 million barrels in 2014, according to data from economist Charles Robertson at investment bank Renaissance Capital.