Nigeria's oil minister said on Tuesday that a boost in US oil and gas production had begun affecting Nigerian exports, with the United States a major market for crude from Nigeria, Africa's biggest producer.
Analysts have previously warned that Nigeria would eventually have to look to different markets to compensate for a fall in exports to the United States, and have predicted its sales could shift toward Asia.
"The world is now becoming even more competitive all around," Oil Minister Diezani Alison-Madueke told an annual conference of industry heavyweights in the capital Abuja.
"US shale oil and an increase in their gas production is already affecting our exports to the United States. Bear in mind that the United States is one of our major importers in this sector."
She said that Nigeria would work to remain competitive, but provided few further specifics.
The International Energy Agency has said the United States could become the world's biggest oil producer by around 2020. Domestic output has been particularly boosted by new techniques like fracking.
Nigeria is Africa's largest oil producer, with a current output of around 2.0 million barrels per day.