Metz defender Sebastien Bassong is trialling with Newcastle United.
The 22-year-old has previously been linked with Premier League rivals Everton.
Keegan is looking to bolster his backline after the summer departures of Stephen Carr, Peter Ramage and David Rozehnal and Bassong could win a deal if he impresses.
The Paris-born player has made 79 appearances for Metz over the last three seasons and scored one goal after graduating from the club's B team. He played 19 times for the club during the last campaign.