The price of crude oil has surged past $120 a barrel.

Supply concerns from Opec members Nigeria and Iran as well as a weaker dollar pushed US crude prices up by $4 to $120.36 before closing at $119.97 on Monday.

While in Iraq, Kurdish rebels threatened suicide attacks against US interests, Royal Dutch Shell PLC said militants struck a facility owned by one of the company's joint ventures in Nigeriar. Shell said some production has been shut down.

Oil's sharp rise this year has driven gasoline prices to unprecedented levels.

In most years, oil prices peak in May or early June, then mostly decline for the rest of the year. But oil at $120 is something to reckon with.