LONDON (AFP) - Crude oil prices slumped Monday on mounting fears of slowing energy demand in the United States after a historic credit rating downgrade, analysts said. Traders also took their cue from tumbling global stock markets, which dived lower on mounting concern over a potential economic downturn. New Yorks main contract, West Texas Intermediate light, sweet crude for delivery in September, plunged $3.33 to $83.55 a barrel. Brent North Sea crude for September shed $3.40 to $105.97 a barrel in late afternoon London deals. Crude oil prices started the week on a negative side ... as investors prompted to a heavy sell off amid worries about weak US oil demand for the medium term, said Sucden analyst Myrto Sokou. After markets closed for business on Friday, Standard & Poors shocked the world as it became the first ratings agency to scrap Washingtons cherished triple-A credit rating. S&P downgraded the United States to AA+ with a negative outlook, citing concerns over its massive debt.