Updated from 11:48 a.m.
Crude oil prices ended lower Monday, after spending much of the session above the $40-a-barrel level.
The benchmark U.S. crude lost 46 cents, or 1.1% to $39.50, while gasoline futures fell 2 cents, or 1.6%, to $1.302 a gallon.
Prices touched a one-month high last week as traders worried about short-term supply issues in Russia and Iraq as well as the possibility of new terror attacks against the U.S. ahead of the presidential election.
Concerns about supplies from Russia come as Iraq once again struggles to restore exports to a normal level following another set of problems with a pipeline feeding southern export facilities.
Prices have jumped almost $5 this month, after closing below $36 a barrel for the first time in three months.
The Organization of Petroleum Exporting Countries is scheduled to meet July 21 to decide on an increase in its production ceiling of a half-million barrels a day in August. The measure was part of a broader agreement reached at OPEC's June meeting, when it decided to increase official production by two million barrels a day in July.
At one point recently, oil prices had fallen 15% from their record high of more than $42 a barrel, reached the day before OPEC's June 3 meeting.
During May, traders bid up prices on short-term supply concerns triggered by strong global demand and terrorist attacks on oil industry personnel and facilities in the Persian Gulf region ahead of the peak summer driving season in the U.S. and Europe.