Thousands of French high school students and workers protested against labour reforms Wednesday, heaping pressure on President Francois Hollande's already unpopular and fractured Socialist government with presidential elections looming in 14 months.

Teenagers and students threw eggs and firecrackers as they marched in Paris chanting slogans such as "El Khomri, you're beat, the youth are in the street" in reference to Labor Minister Myriam El Khomri.

Unions shut down operations at Nigeria's state oil firm Wednesday in protest at restructuring plans, a spokesman said, in a move that could worsen fuel supplies in Africa's largest producer. 

"Operations at the Nigerian National Petroleum Corporation (NNPC) have been grounded nationwide," said Babatunde Oke, of the Petroleum and Natural Gas Senior Staff Association (PENGASSAN). 

Europe's main stock markets inched upward at the start of trading on Wednesday, with London, Frankfurt and Paris each gaining around 0.1 percent.

London's benchmark FTSE 100 index opened at 6,130.03 points compared with Tuesday's close.

German power giant E.ON on Wednesday said it booked a 7.0-billion-euro ($7.7-billion) net loss in 2015 and warned that "the course ahead will be tougher and longer than anticipated."

E.ON said in a statement that "impairment charges of 8.8 billion euros... primarily on generation assets resulted in a substantial net loss of 7.0 billion euros" last year.

French prosecutors have opened a serious fraud investigation into Volkswagen over devices the German automaker fitted into cars to cheat emissions tests, a judicial source told AFP on Tuesday.

Oil prices retreated Tuesday, with Brent crude back below $40 a barrel, after Kuwait reportedly ruled out cutting output, analysts said.

Prices had strengthened earlier in the day, extending recent strong gains won largely on hopes of oil production freezes to ease a global supply glut. 

The South African government said Tuesday that exploration for shale gas will begin in the next 12 months, ending years of speculation over the project.

South Africa's semi-desert Karoo region is believed to hold at least 485 trillion cubic feet of shale gas, but drilling has been delayed by environmental and economic concerns.

World stock markets sank deep into the red on Tuesday, as China released data showing another hefty slump in exports, sparking renewed worries over the nation's powerhouse economy.

China's exports dived more than a quarter on-year in February, new data showed Tuesday, while imports were almost 14 percent off -- far worse than forecast.

Oil prices dipped in Asia Tuesday but Brent crude stayed above $40 a barrel, as traders took profits after solid gains over the past three weeks.

Prices have strengthened following talks of a production freeze, with a producers meeting mooted on March 20 in a bid to ease a global supply glut that has depressed the market.

German industrial production jumped unexpectedly in January, propelled by increased activity in the manufacturing and construction sectors, the economy ministry said on Tuesday. 

The ministry calculated that factory output -- a key yardstick for gauging the health of Europe's biggest economy -- leapt by 3.3 percent in January compared with a month earlier, corrected for seasonal factors.