The price of a barrel of oil surged again on Friday, amid production disruptions in Nigeria and military tensions in the Persian Gulf.

On the New York Mercantile Exchange, oil for June delivery rose $2.46 US to settle at $118.52. It had hit an intraday peak of $119.55 earlier in the day.

The rise came after a ship contracted by the U.S. Defence Department fired flares and warning shots from machine guns at two small unidentified boats in central Persian Gulf. A U.S. navy spokeswoman said the contracted ship was radioed shortly after for an identity check by the Iranian coast guard.

In Nigeria, a rebel group claimed it had staged another attack on a pipeline. The Movement for the Emancipation of the Niger Delta said it attacked the line late Thursday. Shell confirmed that one of its pipelines was hit, but did not provide more details.

The militant group has staged several attacks in recent weeks on Nigerian pipelines.

In addition, workers at a ExxonMobil joint venture in Nigeria began a production cutback to push for more pay.

Upward pressure on oil prices also came from Scotland, where workers at the Grangemouth refinery are due to take part in a two-day strike starting Sunday over pensions. If the strike goes ahead, a major pipeline supplying 40 per cent of Britain's daily oil production would be shut down, the BBC reported.

With files from Associated Press