Brent crude for delivery in August slid 80 cents to USD 114.01 per barrel in London late afternoon deals compared with Friday's closing level.
US benchmark West Texas Intermediate for August dropped 82 cents to USD 106.01 per barrel.
Both contracts had soared late last week to nine-month peaks, buoyed by supply concerns as Iraq was rocked by a violent militant insurgency.
However, the oil market has since pulled lower as those Iraq supply risks failed to materialise, according to analysts.
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Added to the picture, prices have also been dented by recovering crude supplies from Libya, and by profit-taking.
"As yet, no actual disruption to supply has occurred as the majority of Iraq's oil infrastructure remains far removed from the escalating violence," said analyst Dorian Lucas at energy consultancy Inenco.
"With no actual disruption to supply, Brent crude plateaued around $115 ... And has now started to relinquish some of its recent gains.
"This could be attributed to the unrealised fears for supply, but is also likely to have been weighed upon by the recent increase in Libyan crude oil output," he added.