Celtic had Rosenborg left-back Birger Meling scouted during the Norwegian side's Champions League qualifier with Dinamo Zagreb last Wednesday, according to the Scottish Sun.
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The Parkhead side have been heavily linked with a move for Meling for the past few weeks as they seek to provide competition for Boli Bolingoli-Mbombo after the sale of Kieran Tierney to Arsenal.
Even though an offer has yet to materialise, Celtic continue to monitor the player as they weigh up their options before the end of the transfer window.
Manager Neil Lennon has said he still wants "two or three" players to add to his squad before the window closes at midnight on Monday 2 September.
Celtic have added seven new signings in the window, though only three of those are experienced first-team professionals.
Rosenborg lost the first leg of their Champions League play-off opener, going down 2-0 in Zagreb.