Transfer news: Celtic make an approach to Dundee United for Stuart Armstrong

Stuart Armstrong scores the decisive goal as Dundee United beat Celtic in December 2014

Celtic have made an official approach to Dundee United for midfielder Stuart Armstrong, according to Sky sources.

Discussions regarding a fee are ongoing for the 22-year-old, who has 18 months remaining on his contract at Tannadice.

Celtic have already secured the services of Armstrong's team-mate Gary Mackay-Steven, who will move to Parkhead in the summer after signing a pre-contract agreement last week.

Armstrong, who joined Dundee United from Inverness Caledonian Thistle in the summer of 2009, has scored four goals in 20 games for Scotland U21s.