Oil tumbled nearly 3 percent on Friday, with U.S. crude below $100 a barrel for the first time since February, as an abrupt slow-down in U.S. hiring soured economic sentiment and technical triggers intensified selling.

Brent's slide of nearly $3 a barrel took three-day losses to more than 5 percent, the deepest sell-off since December, rattling traders who had been lulled by low volatility this year.

While a downbeat U.S. jobs report weighed, traders said a combination of less definitive factors -- from confusion over margin changes to the breach of Brent's 200-day moving average -- compounded selling. Some dealers were reminded of the abrupt $10 collapse in prices on May 5 last year.

"We have broken through key technical levels here after a disappointing employment report and the PMI number from Europe which suggest that the recovery is stalling and could affect energy consumption," said Gene McGillian of Tradition Energy.

This week's quickening rout has effectively erased any "Iran premium" from the market, suggesting that concerns over a darkening economic outlook were taking precedence over worries about reduced exports from OPEC's second-largest producer.

Prices for international benchmark Brent have tumbled $15 from their 2012 high of $128.40 a barrel, struck on March 1. June Brent futures fell $2.90 to settle at $113.18 a barrel.

Brent dropped as low as $111.76 a barrel before trading on the contract's premium to U.S. futures pushed the spread out to $14.85 a barrel, up $1.30 from Thursday's levels. The Brent/U.S. crude spread ended at $14.69 based on settlements.

U.S. crude dropped nearly 4 percent, off $4.05 to settle at $98.49 a barrel, breaking below $100 for the first time since February. U.S. crude posted a 6.1 percent weekly loss.

Prices are now a tad higher than they were in early November, when a U.N. report on Iran's nuclear program stirred new action against Tehran.

Stock markets also fell, with Wall Street indices down more than 1 percent after data showed that U.S. employers added 115,000 workers last month far less than forecasts for 170,000.

Adding to economic worries, Markit's Eurozone Manufacturing Purchasing Managers' Index (PMI) dropped to 45.9 last month from 47.7 in March, marking its lowest reading since June 2009. <EUR/PMIM>

A second day of deep losses ended a period of unusually calm trading this year. The Oil Volatility Index <.OVX> surged from a historic low earlier this week, rising 25 percent in the past two days, its biggest such gain since last August.

Amid the wave of selling, Brent dipped below 30 on the 14 day relative strength index on Friday, which is traditionally seen as a sign a commodity has been oversold.

Rising U.S. inventories have added to bearish pressure this week, with government data showing crude stockpiles hit the highest level since 1990, while industry data provider Genscape reported stocks at the Cushing, Oklahoma, delivery point for U.S. oil futures hit another record on May 1.

"The story is one of slowing demand, rising supply, and a reduction in the Iranian conflict premium -- that has meant that oil traders have run out of excuses to stay bullish," said Phil Flynn, senior market analyst with PFGBest in Chicago.

"The only really bullish factor lurking there has been the potential for a conflict with Iran."

Iran and major powers resumed talks in mid-April in Istanbul after a gap of more than a year, during which time the United States and European Union stepped up efforts to curb Tehran's nuclear ambitions through new sanctions and an oil embargo which come into effect over the summer. They are to meet again on May 23 in Baghdad.

On Thursday, OPEC Secretary General Abdullah al-Badri said the producer group is working hard to bring down prices that jumped towards $130 a barrel earlier this year, pumping much more than its official target even as exports from member country Iran dwindle.