Barcelona coach Gerardo Martino and captain Carles Puyol have demanded more intensity from the players after Tuesday's 2-1 Champions League defeat at Ajax Amsterdam.

The Group H reverse was Barca's first loss this season in all competitions — and its first under Martino. It cost them a chance to seal top spot in the group, having already secured a place in the last 16.

In the other group match, AC Milan defeated Celtic 3-0 in Glasgow. Kaka and Cristian Zapata pounced on two pieces of poor defending at corners by Celtic to score in the 13th and 49th minutes for Milan, before Mario Balotelli wrapped it up in the 59th.

Barca has 10 points, Milan eight and Ajax is third with seven. Milan can secure passage to the knock out stage with a draw against Ajax.

A fired-up Ajax, roared on by their vocal fans, took the game to an injury-hit Barca in the first half, harrying and hassling them before taking a deserved 2-0 lead with goals from Thulani Serero and Danny Hoesen.

Although the Spanish champions dominated after Ajax were reduced to 10 men shortly after halftime and pulled a goal back through Xavi's penalty, they struggled against a home defense well drilled by coach and former Barca player Frank de Boer.

"Ajax began with more intensity than us and we paid the price," said Martino.

"The second half was a different story and we played in a way more in tune with what Barca represents.

"The problems in the first half came more when we had the ball than when we didn't."

Puyol said the team would have no chance of adding to their four European crowns if they started every match so lethargically.

"The idea is to have possession of the ball but when you are up against a team that also knows how to play and is full of fight this is what happens," he said. "If you do not have enough intensity they will roll right over you."

"You have to gain experience from all your matches and if we want to win titles we have to improve," he added.

Puyol's defensive partner Gerard Pique said it was Barca's worst first-half performance of the season.

"We played very badly and we were lacking in commitment," added the Spain international. "In the second half, although we improved, we did not create enough chances to get the second goal," Pique said.

Barca's next match is at La Liga rivals Athletic Bilbao on Sunday. However, Barca will once again be without Lionel Messi and goalkeeper Victor Valdes, who are both injured.