The situation in Nigeria is the biggest upside risk in the oil markets, strategist Helima Croft said Monday.
The country's oil output has fallen to its lowest in decades following several acts of sabotage by militants.
"If Nigeria goes offline, it's sticky. These armed militants are very intent on shutting down production. They have the capacity to do so," Croft, global head of commodity strategy at RBC Capital Markets, said in an interview with CNBC's "Power Lunch."
"I don't think we should write them off because they are armed to the teeth."
Oil prices surged Monday on the growing Nigerian oil output disruptions and a new outlook by Goldman Sachs that said the market had ended almost two years of oversupply and flipped to a deficit.
U.S. crude futures settled up $1.51 at $47.72 a barrel, at their highest since Nov. 3, when they closed at $47.90.
Brent crude futures were trading at $48.99 per barrel, up $1.13, or 2.36 percent.