MILAN (AFP) - AC Milan beat Celtic 1-0 here on Wednesday to book their place in the quarter-finals of the Champions League, but they needed extra-time before breaking the Scottish side's brave resistance.

AC Milan's Ricardo Kaka celebrates after scoring against Celtic during their Champions League soccer match first knock-out stage, second leg in Milan March 7 2007. [Reuters]

Celtic were under almost constant pressure from the first whistle, but what they lacked in technique they more than made up for in spirit and held the six-time European champions to a goalless draw after 90 minutes, leaving the last 16 tie deadlocked at 0-0 on aggregate.

But after repelling attack after attack, the visitors, roared on by a raucous army of travelling supporters, finally succumbed in the third minute of the first-half in extra-time.

Kaka, who had seen a shot come back off the bar in normal time, ran from his own half at a tiring Celtic defence and evaded a late lunge by Stephen McManus before sliding the ball past Artur Boruc.

Kaka insisted his side were worthy of a place in the last eight given their performances over the two legs.

"Celtic are an experienced team and they put us in difficulty, but if you look at the two games, we deserved to go through," he said.

"I am delighted with what I did and what the team has achieved. I'm going home happy."

AC Milan coach Carlo Ancelotti admitted Celtic had given his team a real battle.

"We knew it would be tough, but I thought we coped with it well," he said.

"We gave away little and created a lot. We suffered at times, but the joy of going through makes it worthwhile."

Asked if he thought Milan could win the Champions League, he replied: "With Real Madrid, Inter Milan and Barcelona out, it shows that there are no guarantees in the Champions League.

"Anyone of the eight teams left has a chance. It's very open."

Celtic coach Gordon Strachan had nothing but praise for his gallant players.

"If there was a Champions League trophy for sheer guts and determination, we'd win it," he said.

"We must improve our technique if we want to push forward, but we are certainly on the right road.

"The tournament has been a great experience for us and we will learn from it."

Milan's starting line-up included seven of the team that played in the 2003 Champions League final in which they beat Italian rivals Juventus on penalties.

Milan came close to taking the lead in the fourth minute when Clarence Seedorf fed Kaka in space, but the Brazilian's curling right-footed shot was clawed away by Boruc's fingertips.

Daniele Bonera came to Milan's rescue 60 seconds later, the defender's last-ditch block denying Jiri Jarosik an almost certain goal.

Kaka then spurned a golden opportunity when he dragged a shot wide after an incisive run from his own half which had left two Celtic defenders for dead.

Milan maintained the pressure at the start of the second half with Seedorf screwing an effort wide and Andrea Pirlo driving a free-kick straight into Boruc's grateful hands.

Milan were making all the running, but a rare moment of relief for Celtic came when Jarosik stole possession and cleverly released McManus down the left. The tenacious midfielder burst towards the area, but his low cross was easily intercepted.

Jarosik made way for combative Danish midfielder Thomas Gravesen as Strachan looked to bolster his midfield, before Milan veteran Filippo Inzaghi saw his flicked header cleared off the line in the 70th minute.

Paolo Maldini went agonisingly close after Boruc had spilled his header, but the Milan captain narrowly failed to get his toe to the loose ball.

Celtic than had claims for a penalty turned down when Shunsuke Nakamura, who spent three years in Italy with Serie A side Reggina, went down after colliding with Massimo Ambrosini.

Boruc produced a superb save to stop Seedorf's drive from finding the top corner, before Kaka's sublime chip struck the Celtic crossbar in a frantic end to normal time.

Milan, who had 25 goal attempts in the 90 minutes, finally found a way through when Kaka drove at a backpedalling Celtic defence and coolly finished.