Oil prices have hit levels not seen in nearly a year, threatening drivers with another round of increases at the gas pump.

Crude oil prices have topped $70 per barrel for the last week, pushed higher by violence against oil workers in Nigeria and fears of tightening supplies worldwide. Contracts to buy crude for future delivery closed Friday at $72.81 on the New York Mercantile Exchange, the highest price since last August. They slid 0.8 percent Monday to close at $72.19.

Rising crude prices usually mean that motorists will soon pay more for gasoline. The effect isn’t immediate and can take weeks to be noticeable. But there are signs it is already happening.

The national average price for gasoline peaked at $3.23 for a gallon of regular in late May and then started tumbling. But the drop stalled around the start of July. In recent days the national average has started to rise, although by mere fractions of a penny. It now stands at $2.97, according to the AAA auto club.

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