Investing.com -- The S&P 500 energy index fell 2.1% on Monday as crude oil prices declined following diplomatic developments in the Middle East.

Brent crude futures dropped 6.2% to $106.45 per barrel, while US West Texas Intermediate crude fell 6.5% to $97.40 per barrel. The price decline came as talks between Iranian and Qatari officials raised expectations of reduced supply disruptions in the Middle East. Iran's president said the supreme leader granted the government certain powers to pursue peace negotiations.

Energy majors ExxonMobil (NYSE: XOM) and Chevron (NYSE: CVX) fell 2.4% and 2%, respectively.

Valero Energy (NYSE: VLO), Occidental Petroleum (NYSE: OXY), and Devon Energy (NYSE: DVN) declined between 3.1% and 3.9%, ranking among the top percentage losers on the energy index.

ConocoPhillips (NYSE: COP) dropped 2%, EOG Resources (NYSE: EOG) fell 2.3%, and Targa Resources (NYSE: TRGP) declined 2.6%.

Oilfield services companies also moved lower, with Halliburton (NYSE: HAL) down 2.5% and SLB (NYSE: SLB) falling 2.3%.

Refiners Phillips 66 (NYSE: PSX) and Marathon Petroleum (NYSE: MPC) declined 2.2% and 2.6%, respectively.