Brent crude, the international benchmark for oil, was down 33 cents from its previous close at $55.55 per barrel at 11:38 a.m. EDT (1538 GMT). Earlier in the session, Brent had climbed to its highest since March 7 at $56.16.
US West Texas Intermediate (WTI) fell by 19 cents to $52.88 a barrel, after touching a five-week high of $53.23.
Brent has risen in each of the previous six sessions, while WTI gained for the last five days.
Analysts said there are worries demand growth could falter, and other indicators were warning that the market had not yet cleared enough of its surplus to keep prices rising.
"There's a lot of heightened geopolitical tension on two fronts," said Phil Streible, senior market strategist at RJO futures in Chicago. Rising concerns about North Korea and Syria may depress oil demand, he said.
North Korean state media warned on Tuesday of a nuclear attack on the United States at any sign of American aggression as a US Navy strike group steamed toward the western Pacific.
US President Donald Trump said in a Tweet North Korea was "looking for trouble" and the United States would "solve the problem" with or without China's help.
"Geopolitical tensions are bad for global demand growth," said Olivier Jakob, managing director of Petromatrix, noting rising acrimony between the United States and North Korea.
He also said the widening discount of the current Brent crude price to the contract in the next month is "basically telling you the market is not actually that tight".
US crude inventories have touched record highs at the US storage hub of Cushing, Oklahoma and in the US Gulf Coast in recent weeks, according to US government data.
A Reuters poll of analysts forecast a rise in US crude inventories for a fourth straight week.
Data from industry group API is due out on Tuesday, while figures the US Energy Information Administration will be released on Wednesday.
Several factors still offered support to oil prices.
Russian Energy Minister Alexander Novak said his country's output cuts would reach 250,000 barrels per day (bpd) by mid-April, TASS news agency reported. Another shutdown at Libya's largest field, Sharara, also kept oil off the market.
Russia was part of a deal between the Organization of the Petroleum Exporting Countries and other producing nations to cut output by 1.8 million bpd in the first six months of 2017.