Strong negative sentiment from some OPEC's top oil producers pushed crude prices deep into negative trading on the last Monday of 2015.

Crude oil prices rallied last week on signs some of the excess crude oil supply was moving back into the market. A holiday-shortened rally was supported in part by a warning last week that Westerners may be the target of violence in parts of China.

Oil prices plummeted in early Monday trading after Iran and Saudi Arabia both signaled the weak market would last. Brent crude oil lost more than 2.3 percent to start the trading day in New York at $37.01 per barrel. West Texas Intermediate, the U.S. benchmark price for crude oil, lost 2.6 percent, but defended its premium against Brent to trade at $37.11 per barrel.

Iran this week stood by its position to add more oil to the global market after sanctions pressures ease next week. Officials with the National Iranian Oil Co. said Iran would add more barrels to the market, saying low production costs in Iran made it feasible even during the era of lower oil prices.

Saudi Arabia, meanwhile, said it posted a record budget deficit, which it would counter with spending cuts and a five-year plan to reduce its economic exposure to oil. Riyadh recorded a $97.9 billion budget deficit in 2015. Russia's energy minister blamed Riyadh for the weakened crude oil market.

Concerns over national security in Iraq, a member of the Organization of Petroleum Exporting Countries, eased somewhat Monday after the Iraqi military declared parts of Ramadi, in restive Anbar province, were liberated from the Islamic State, a Sunni jihadist group.

Crude oil prices are down about 13 percent for the month. OPEC said it expected balance between supply and demand would return to the market during the latter half of 2016, a sentiment backed Monday by Kremlin officials.