By Eric Yep 
 

Crude-oil futures swung between gains and losses in Asian trade Thursday as investors assessed a lower growth target from China, a slower increase in U.S. oil stockpiles and ongoing production issues in Libya.

On the New York Mercantile Exchange, light, sweet crude futures for delivery in April traded at $51.82 a barrel at 0419 GMT, up $0.29 in the Globex electronic session. Brent crude for April delivery on London's ICE Futures exchange rose $0.14 to $60.69 a barrel.

China on Thursday projected economic growth of about 7% for 2015, lower than the 7.4% it achieved in 2014. Beijing also announced several energy-related goals, including reducing energy intensity--the amount of energy needed to increase gross domestic product--by more than 3.1% for 2015, strictly controlling energy use and boosting clean-energy sources like hydropower.

A softer Chinese economy has been mostly priced in by the market considering that crude demand was artificially inflated by stockpiling in recent months, analyst Daniel Ang at Phillips Futures said.

He said investors are mainly focused on the premium of Brent over Nymex crude and the impact of the refinery workers' strike in the U.S. It may take a few days for the full impact of China's growth forecasts to be felt, he said.

"Traders are taking positions thinking that the Brent-WTI spread will narrow, and that's a bigger issue for oil now," Mr. Ang said.

U.S. refiners and striking union workers are digging in for a protracted battle that could last through the spring. The strike that affects around 20% of U.S. oil-refining capacity, along with high inventories have widened the Brent-WTI spread, which is currently around $8.85 a barrel.

Oil had moved in opposite directions overnight with Nymex crude settling 2% higher as U.S. stockpiles rose less than expected last week, while Brent lost 0.77%.

Meanwhile, Libya's National Oil Co. on Wednesday declared a force majeure related to 11 oil fields in the center of country, saying it was no longer able to ensure security at the sites after attacks by Islamic State militants.

In Switzerland, Iran and six major powers neared an understanding on a deal over Tehran's nuclear program, on condition that Tehran refrain from amassing enough fuel for a nuclear weapon for at least a year.

Saudi Arabia's oil minister Ali al-Naimi Wednesday made some of his clearest statements yet on its oil policy.

"Today, it is not the role of Saudi Arabia, or certain other OPEC nations, to subsidize higher cost producers by ceding market share," he said. Saudi Arabia has very low production costs and is more efficient than other producers, he said. "It is an advantage which we will use, as any producer would, to help supply dependent global customers," Mr. Naimi said.

Nymex reformulated gasoline blendstock for April--the benchmark gasoline contract--rose 6 points to $1.9263 a gallon, while April diesel traded at $1.9003, 10 points lower.

ICE gasoil for March changed hands at $582.75 a metric ton, up $5.50 from Wednesday's settlement.

--Alison Sider, BenoƮt Faucon and Mark Magnier contributed to this report

Write to Eric Yep at eric.yep@wsj.com