Oil prices fell on Friday, pressured by economic uncertainty ahead of a possible debt deal in Greece, concerns about China's sluggish manufacturing sector, and weak U.S. petroleum demand.

China's manufacturers had a sluggish start to the year, a survey of purchasing managers showed, weighing on both oil and copper prices.

News that major powers seeking to negotiate an end to Iran's suspected pursuit of nuclear weapons are soon to lay out what Tehran would need to do return to talks added to pressure on oil prices, analysts and brokers said.

There are questions about Europe as far as [oil] demand and yesterday's weak U.S. gasoline demand numbers indicate the economy may not be doing as well as thought, said Phil Flynn, an analyst at PFGBest Research in Chicago.

Signs of slowing in Chinese manufacturing didn't help, and there are signs that there are efforts to take some of the tension out of the air on Iran, Flynn added.

In London, ICE Brent March crude settled at $109.86 a barrel, sliding $1.69, or 1.52 percent, having fallen intraday to $109.42, below front-month Brent's 100-day moving average at $109.62. For the week, front-month Brent fell 58 cents, or 0.53 percent.

U.S. February crude settled at $98.46, dropping $1.93, having fallen intraday to $97.91, pushing below the 50-day moving average at $99.07. For the week, front-month U.S. crude dipped 24 cents.

U.S. crude's deficit to Brent widened to $11.53 at the close, from $11.01 on Thursday.

Total Brent trading volume slipped 12 percent from its 30-day average, according to Reuters data. Total U.S. crude dealings were up 22 percent from the 30-day average.

A planned reversal of the Seaway crude oil pipeline in the United States is being delayed two months to June 1, pushing back near-term expectations that the U.S. Midwest crude oil glut will be eased.

U.S. gasoline and heating oil both weakened after Thursday's weekly oil data from the Energy Information Administration showed rising gasoline and total distillate stockpiles and weak demand.

U.S. gasoline demand last week plummeted to the lowest level in more than a decade, the EIA data showed.

Petroleum consumption in December declined 5.9 percent versus December 2010 and total 2011 demand fell 1.2 percent to an average of 18.9 million barrels per day compared with the previous year, the American Petroleum Institute said on Friday.

Iran and Sanctions

Japan pledged to keep cutting purchases of Iranian crude, the clearest public offer of support yet among Asia's big buyers for U.S. efforts to get consumer nations to stop buying Iran's oil.

European Union foreign ministers will assure Greece on Monday that it will still be able to buy oil on reasonable terms after the introduction of a planned EU ban on Iranian crude, a senior Brussels official said.

EU ministers meet on Monday and are expected to announce sanctions on importing Iranian oil.

The expected statement on what terms talks with Iran could be started by major powers Britain, China, France, Germany, Russia, and the United States would be the latest signal the diplomatic path remains open to Iran despite tougher sanctions and speculation of a military strike on its nuclear facilities.

An ally of Iran's supreme leader called for Israel to be punished for killing a nuclear scientist.

French President Nicolas Sarkozy said that time was running out to avoid a military intervention. He appealed to China and Russia to support new sanctions. Russia and China are veto-wielding U.N. Security Council members that have been reluctant to back an oil embargo, let alone military force.

(Additional reporting by Gene Ramos in New York, Jessica Donati in London and Manash Goswami in Singapore; Editing by David Gregorio and Marguerita Choy)