Crude oil prices started Friday holding relatively steady despite a downbeat forecast from the International Energy Agency.

Brent crude oil prices started the day relatively unchanged from Thursday's close to $58.53 per barrel. West Texas Intermediate, the U.S. benchmark, was steady at $52.65 per barrel.

Crude oil markets were volatile for the first full trading week in July as investors took stock of Greek economic troubles and ongoing nuclear negotiations with Iran and its international partners. Deadlines and key developments on both fronts may be settled during the weekend.

Greek's economic problems were in part behind the series of events that undermined global performance during the latest fiscal crisis, though European leaders said the region's economy would weather the storm this time around. More Iranian crude oil could find its way to an already oversupplied market if sanctions pressure eases as a result of a final deal in Vienna, though a full market return may be a ways off.

A weak global economy and gains in U.S. oil production are serving as a throttle on crude oil price momentum. Price levels for Friday are more than 40 percent below June 2014 figures.

The International Energy Agency said in its monthly market report global oil demand growth is expected to slow by 14 percent next year to 1.2 million barrels per day.

"World oil demand growth appears to have peaked in the first quarter at 1.8 million bpd and will continue to ease throughout the rest of 2015 and into 2016 as temporary support fades," the Paris-based agency said.

In terms of production, output from the Organization of Petroleum Exporting Countries is at a three-year high with non-OPEC production expected to lag, IEA said.

The downbeat assessment from IEA follows a trimmed forecast for global economic growth by the International Monetary Fund. Slow growth from the United States, Greek debt concerns and a recent crash in the Chinese stock market are all short-term concerns, IMF Chief Economist Olivier Blanchard said.

"If I had to summarize, I would say please do not focus too much on the Greek events and China stock market events," he said. "The important thing is moderate growth continues."