Manchester United have been following Napoli star Dries Mertens "very closely" according to Pierpaolo Marino, the Serie A side's former sporting director.

Mertens has transformed into one of the most lethal centre forwards in Italian football over the last 18 months after being converted from a winger following the sale of Gonzalo Higuain to Juventus.

Last season he finished the campaign as one of Europe's most productive forwards, scoring 34 goals – 28 of them in the league – while providing 15 assists, and has already netted nine times in all competitions this term, laying on another five.

That form caught the attention of Chelsea manager Antonio Conte, who was keen to bring the Belgium international to Stamford Bridge this summer, as revealed by the player himself. But having been rewarded with a new contract at the Stadio San Paolo, he opted to remain in Naples.

Mertens' new deal does include a buyout clause which stands at €28m for clubs outside of Italy, according to Gazzetta dello Sport, a relatively paltry fee that is unlikely to prevent Premier League clubs from circling.

And according to Marino, Jose Mourinho's side are one of them, telling Tuttosport: "It turns out United are following him very closely."

United's interest in Mertens was also reported by Belgium's La Dernière Heure earlier this year, who also named Tottenham Hotspur as another club closely monitoring his situation.

But According to Het Laatste Nieuws, Chelsea boss Conte is a firm admirer of Mertens, having identified him as an ideal addition to his attack given his versatility and potent scoring touch.

Mertens' sensational form has helped Napoli take an early lead in Serie A, leading Juventus by two points and hoping to end the Bianconeri's stranglehold on the Italian top flight.