Oil has been found off the coasts of Liberia and Sierra Leone, energy companies have announced.
African Petroleum and Anadarko say further tests are needed to see how commercially viable the finds are.
Nevertheless, hopes have been raised that an oil bonanza could spur growth in the West African states - still recovering from civil wars.
Drillers rushed to the region five years ago when one of Africa's biggest oil fields was discovered off Ghana.
Liberia and Sierra Leone have been eager to develop their mining and energy industries after years of conflict hindered investment and left infrastructure in ruins.
The government of Sierra Leone issued a statement congratulating Anadarko.
This is the second oil field Andarko has found off Sierra Leone.
Liberia has also welcomed the latest discovery - but urged patience.
"It will take time to fully appraise this discovery," Liberia National Oil company head Randolph McClain said on local radio, according to Reuters news agency.
"And years - between five to seven years - before a drop of oil is produced from the well."
Further west, Nigeria is Africa's biggest oil producer.