New York's main contract, West Texas Intermediate crude for delivery in February, gained three cents to $98.73 in morning trade.
Brent North Sea crude for February delivery was up 37 cents to $110.81 on its last trading day.
"Any shortage of supply caused by a disruption in Nigeria is likely to cause a direct spike on crude prices," said Ker Chung Yang, commodities analyst at Phillip Futures in Singapore.
Nigeria's labour unions today decided to call off street protests due to security concerns, but said a week-old nationwide strike over soaring fuel prices will remain in place.
The announcement came after talks between the government and labour leaders on Thursday and Saturday with President Goodluck Jonathan failed to find a compromise.
Nigeria is Africa's biggest oil producer.
Traders are also closely watching the situation in the eurozone, where the debt downgrade of nine nations could lead to a dampening of crude oil demand, analysts said.
The cuts on Friday by Standard & Poor's reignited a debate over boosting the bloc's bailout fund, which is now considered too small to rescue all of its troubled economies.
S&P said it downgraded France's top AAA rating by one notch to AA+, with a negative outlook, but left European powerhouse Germany unchanged at AAA, reflecting its stronger economy and finances.