OUAGADOUGOU, Burkina Faso — Russia has been steadily expanding its military influence across Africa, alarming Western officials with increasing arms sales, security agreements and training programs for unstable countries or autocratic leaders.

In the Central African Republic, where a Russian has been installed as the president’s national security adviser, the government is selling mining rights for gold and diamonds at a fraction of their worth to hire trainers and buy arms from Moscow. Russia is seeking to ensconce itself on NATO’s southern flank by helping a former general in Libya fight for control over his government and vast oil market.