After closing the previous session at $70.98 a barrel on Thursday, global benchmark Brent crude gained 0.2% to trade at $71.12 per barrel at 0630 GMT.
These concerns outweigh the optimism derived from the ongoing OPEC supply cuts and the United States sanctions on Iran and Venezuela that helped the prices reach five-month tops earlier this week.
Oil markets have been lifted by more than 30 percent this year by supply cuts led by the Organization of the Petroleum Exporting Countries and US sanctions on oil exporters Iran and Venezuela, plus escalating conflict in OPEC member Libya.
"We maintain our forecast of 1.4 Mmbpd (in 2019 oil demand growth), but accept that there are mixed signals about the health of the global economy, and differing views about the likely level of oil prices", IEA said. Demand for OPEC crude in 2018 averaged 31.35 million bpd.
Brent and WTI have risen by around 30 and 40 percent respectively since the start of the year.
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After a production glut lead to prices dropping previous year, OPEC members and allies including Russian Federation agreed in December to trim production. Demand is expected to grow by 1.4 percent per day.
OPEC and its allies led by Russian Federation are due to meet in Vienna on June 25-26 to set their policy.
US oil giant Chevron Corporation said on Friday it will buy Anadarko Petroleum Corporation in a stock and cash transaction valued at 33 billion USA dollars to bolster its position in shale and the liquid natural gas market.
Saudi Arabia can add more oil to the market without adjusting production quotas since the kingdom's output in March was some 500,000 bpd below its OPEC target, the source added.
The rig count fell for the past four months as independent exploration and production companies cut spending on new drilling to focus on earnings growth instead of increased output. Additionally, U.S. crude output remained at a record 12.2 million barrels per day.
Nigerian production increased by 11,000 bpd in March to 1.733 million bpd and fell by 5.000 bpd to 1.685 million bpd according to direct communication, according to the monthly oil market report for April.