Global crude oil prices drifted lower in Monday trading on word multilateral talks with Iran failed to find traction as a negotiating deadline expired.

Negotiators in Vienna were unable to reach a comprehensive agreement aimed at resolving the impasse over Iran's nuclear research program. British Foreign Secretary Philip Hammond said progress had been made, though it was "not possible to get an agreement by the [original] deadline."

Brent crude oil prices moved above the $80 mark in recent trading days, though word of an extension to June for negotiations with Iran pushed the global benchmark price down by about 50 cents to $79.90 for the January contract.

Iran secured modest relief from sanctions one year ago during multilateral talks. Under the terms of an agreement, Iran can export about 1 million barrels of oil per day if it maintains its commitment to curb nuclear activity.

It's unclear how sanctions will be enforced following Monday's agreement from Vienna.

Iranian Brig. Gen. Mohammad Reza Naqdi, commander of the Basij military unit, struck a bellicose tone before word of extended negotiations emerged Monday.

"To hell with (the world powers) if they don't want to reach an agreement (with Iran) and want to impose sanctions," he said. "Let us cripple them by closing the oil valves."

Iran loses out on a potential increase in oil revenue through the delay, though Tehran had said it was working to diminish the role of oil in the national economy.

Iran, along with other members of the Organization of Petroleum Exporting Countries, reported a modest decline in overall oil production. The status of the nuclear talks will likely sway ministerial conversations at this week's OPEC conference.

OPEC ministers are mulling their response to the increase in oil production from the United States.

West Texas Intermediate, the U.S. benchmark, followed Brent's lead Monday to trade at $75.90 for the January contract, down about 60 cents from the previous session.