Lautaro Martinez shot Inter Milan to a 1-0 derby triumph over crumbling champions AC Milan on Sunday but his team stayed 13 points behind Serie A leaders Napoli who strolled to a 3-0 win at Spezia.

Argentina forward Martinez headed home his 12th league goal of the season in the 34th minute at the San Siro to make sure that Inter remained in second place and plunge their local rivals even deeper into crisis.

Martinez has been given the Inter captaincy as Milan Skriniar wants to leave for Paris Saint-Germain as a free agent in the summer

"Wearing the armband makes me very proud," he told DAZN.

"Ever since coming to Inter, I have tried to give my best, and this is a special game. We did it again."

A third straight league defeat leaves Milan in sixth and in a downward spiral that threatens to completely ruin a season which less than a month ago looked to be going more or less according to plan.

Stefano Pioli's side have earned just five points from their six league matches since the World Cup and have also been knocked out of the Italian Cup and beaten by Inter in the Super Cup.

Milan have to take on Tottenham Hotspur in their first Champions League last-16 tie since 2014 in less than 10 days.

Having already been thumped twice by Chelsea this season their chances of advancing in Europe also look slim.

They didn't have a single shot on target in an underwhelming derby which was largely dominated by Inter, the nominal hosts on Sunday night, having changed formation and dropped Rafael Leao in order to stop conceding goals.

"We have been very solid at the back in recent weeks but we didn't want to play defensive football. That was our mistake," said Pioli.

"If we'd tried to play our true football we'd have only been in more difficulty."

Martinez could easily have had more than just the one goal which took his calendar year tally in all competitions to seven as he forced two smart saves from Ciprian Tatarusanu and headed a Milan Skriniar cross just wide before he opened the scoring.

He also had a late goal ruled offside by the tiniest margin, his forehead just beyond the defenders before he poked under Tatarusanu, but it made no difference to a curiously comfortable derby win.

Osimhen strikes twice

Victor Osimhen and Khvicha Kvaratskhelia were again the stars of the show for Napoli as they came through at Spezia to briefly extend their advantage to 16 points.

Wing wizard Kvaratskhelia put Napoli ahead from the penalty spot two minutes after the break on the Italian Riviera before Osimhen netted twice, taking his league tally to 16.

Nigeria forward Osimhen pounced on a defensive error to double Napoli's lead in the 68th minute and five minutes later rolled home an easy finish after Mattia Caldara passed straight to Kvaratskhelia on the edge of his own area.

"All the victories are important, we need to keep the momentum and try to win all games," said Osimhen to Sky in Italy.

"It is not going to be easy of course, Serie A is a very competitive league and we have to be on top of our game. We go back, we prepare for the next game, that is the most important thing."

Osimhen ended the day on top of the world after beginning it by going into the stands at the Stadio Picco to apologise to a woman fan who he hit with an off-target pre-match practice shot.

The 23-year-old hugged the woman and posed for selfies before going on to continue Napoli's unstoppable looking title charge.

"I think she was looking at her phone, the ball hit her in the face and I felt so sorry, as I didn't mean to do that," added Osimhen.

"I had to show my appreciation and say sorry for that."

Spezia stay 17th, five points above Verona who are just inside the relegation zone and host Lazio on Monday.