Internacional have confirmed the bid this week.
"We received an 18m verbal offer from Napoli, according to the player's agent," director of football Luis Cesar Souto de Moura told Lancenet.
"We don't accept that this is Leandro Damiao's value. We are certain he will play the 2014 World Cup with Brazil's national team.
"We have not had any talks about setting a value as we don't intend to sell the player. The offer that was presented to us was lower than what he is worth."