LONDON--Oil prices fell on Friday pressured by a rising dollar ahead of data on U.S. drilling activity capping a volatile week of trading.

Brent crude for delivery in May fell 0.4% to $56.34 a barrel on London's ICE Futures exchange. On the New York Mercantile Exchange, May-dated light, sweet crude futures slid 1.2% to $50.20 a barrel.

Oil-field-services firm Baker Hughes will release its weekly U.S. oil rig count, a proxy for activity in the industry. The number of oil drilling rigs has declined for 17 straight weeks and stood at 802 last week, half of what it was just six months ago. The rate of decline, however, has slowed in recent weeks while oil production in the U.S. has been resilient.

The U.S. dollar strengthened on Friday pressuring oil which is priced in dollars and becomes more expensive for holders of other currencies as the greenback appreciates. The Wall Street Journal Dollar Index, which tracks the buck against a basket of other currencies, rose 0.2%.

Oil prices have risen for three of the past four trading sessions, and investors are keeping a close watch on U.S. and OPEC oil production levels, unrest in the Middle East and the Iranian nuclear deal.

In his first public comments after last week's agreement on a framework for a nuclear deal, Iran's supreme leader Ayatollah Ali Khamenei said on Thursday that the U.S. and its negotiating partners must lift all sanctions on his country immediately after a final deal is signed. The West has been demanding a phased repeal of the penalties, conditional on Iran's continuing compliance.

Oil markets are tracking the negotiations because a deal could pave the way for more Iranian crude flooding the already oversupplied global market. In 2011, the country produced about 3.6 million barrels of oil a day and analysts estimate that if sanctions are lifted, Iran could add between 0.5 million and 1 million barrels a day to the already oversupplied global market.

Next week, the International Energy Agency and the Organization of the Petroleum Exporting Countries will publish their monthly assessment of oil markets.

"We expect any rebound in crude oil prices to occur from the third quarter of 2015," UOB Asset Management said in a quarterly report.

Nymex reformulated gasoline blendstock for May--the benchmark gasoline contract--fell 0.2% to $1.7551 a gallon, while ICE gasoil for April changed hands at $527.50 a metric ton, down $0.75 from Thursday's settlement.

Eric Yep contributed to this article.

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