The pair have banged in a combined total of 17 goals in 32 games this season to take the club to seventh in Holland's top flight.
John, 18, was born in Liberia but also holds Dutch nationality, while N'Kufo hails from Congo but has Swiss citizenship.
N'Kufo is currently on a season-long loan from Hannover, and City are said to have enquired about the pair's availability.