A union leader threatened Tuesday to extend a four-day nationwide strike in Nigeria that has helped push world oil prices to record highs.

Adams Oshiomhole, leader of the Nigeria Labor Congress that is spearheading the strike to protest rising fuel prices, said it could be extended if serious violence breaks out, if more union leaders are arrested or if the government refuses to discuss a decrease in fuel prices.

“Once they begin shooting, and once they begin to kill, the character of our struggle will change,” Oshiomhole said, referring to security forces. “We have reserved the right to elongate it. That decision will be taken by Thursday.”

The general strike, which began Monday, was supposed to last four days and resume again in two weeks if the government does not reverse fuel price hikes. The move already has shut down businesses across the country and helped push up world oil prices, with crude reaching all-time highs Tuesday, surpassing $54 per barrel.

Sporadic protests in support of the strike continued across Nigeria.

At least one person was killed Tuesday in strike-related violence when police shot at stone-throwing protesters who burned tires on a road into the southern oil industry center Port Harcourt, rights activist and residents told The Associated Press, citing witnesses. A police spokeswoman in the city said she was not aware of the incident.

On Monday, a 12-year-old boy was killed in a clash between police and demonstrators in the northern city of Kano, police said.

So far, the strike has not disrupted Nigeria’s exports of 2.5 million barrels a day. Nigeria is Africa’s largest and the world’s seventh-largest oil exporter. It also is the fifth-biggest source of U.S. oil imports.

However, oil giant Royal Dutch/Shell said it was cutting production by 20,000 barrels per day following a leak and a fire on a major pipeline transporting crude oil to its export terminal in the Niger Delta.

It was not known what caused the leak on Monday at Moghor in the Ogoni district of the oil-rich delta. Oil industry officials frequently blame pipeline ruptures on vandals and criminal gangs.

While Shell officials tried to investigate, a group of unidentified saboteurs set the pipeline on fire, Shell said in a statement.

Shell said its exports had not been affected, but the company will divert crude oil to another pipeline to enable repairs, a maneuver requiring a cutback in production “of a few days” while the ruptured pipeline is fixed.

Other multinationals said production was normal.

“Production is not affected,” said a Udom Inoyo, a spokesman for Mobil Producing Nigeria Unlimited, a subsidiary of ExxonMobil, the second biggest producer in Nigeria.

Streets in the normally bustling commercial capital of Lagos, home to 13 million, were deserted for a second day as police with assault rifles guarded major intersections.

Other major cities were shut down by the strike, although taxis and many private businesses were still open in the capital, Abuja.

In Lagos, protesters blocked several major roads, smashing windshields of vehicles on the streets to keep people home.

“More than 100 boys attacked us,” said businessman Bright Ezeocha, who sustained a cut on his face when his car windshield was smashed by protesters in the Ojo-Alaba district of Lagos.

The strike also spread Tuesday.

In the southeastern city of Onitsha – a massive bazaar selling everything from locally made car parts to groceries – was shut down after union leaders stepped in.

Nigeria’s fuel market was deregulated last year, prompting a series of price hikes that sparked widespread protests.

Union leaders are demanding that the government reverse hikes last month that pushed up the price of fuel from about $1.19 to $1.50 a gallon.