The game was due to be played in December but had to be postponed because of Milan's involvement in the Intercontinental Cup final which they lost on penalties to Argentine side Boca Juniors.

Brazil international Kaka gave Milan the lead with a superbly-taken strike before Siena's Fernando Menegazzo had a controversial penalty saved by Dida just before half-time.

Danish striker Jon Dahl Tomasson doubled Milan's advantage from close range shortly after the break only for Norway's Tore Andre Flo to ensure a frantic finale by slotting home his sixth goal of the season three minutes from time.

Milan lead the championship on 45 points from 18 matches with Roma on 43 and title-holders Juventus on 40.

Siena, who were playing a league match at the San Siro for the first time in the Tuscan club's 100 year history, remain in 10th place.

Milan coach Carlo Ancelotti played down the importance of taking pole position with a little less than half of the season left.

"Our position in the table is important but not definitive," he said.

"It's important that we maintain this kind of form until the end of the season and hopefully come May we will be in with a chance."

Tomasson admitted Siena, who are still to win away from home this season, made life extremely difficult for the Champions League holders.

"It was very tough and had they scored the penalty it could have been even harder," he said.

"But we expect games like this and they will only get tougher now that we are top. Everyone will want to shoot us down."

Roared on by an expectant home crowd, Milan started positively but too often ran out of ideas in the final third of the pitch where they are normally so effective.

Their first shot did not arrive until in the 24th minute when Andrei Shevchenko's angled drive was beaten away by Generoso Rossi in the Siena goal.

But Milan kept plugging away and were rewarded for their efforts in the 38th minute thanks to a moment of genius from Kaka.

Shevchenko layed the ball off to the quick-footed Brazilian who brilliantly side-stepped Stefano Argilli to create space for a shot and coolly side-footed past Rossi from 15 yards.

Just when it looked as if the floodgates might open, Siena were awarded a dubious penalty.

Defending his own goal, Paolo Maldini appeared to be fouled by Flo when the pair challenged for a high ball and retaliated by pulling down the lanky Norwegian striker.

But the referee saw things differently and incurred the wrath of the home crowd by pointing to the spot and showing a dumbfounded Maldini a yellow card.

Milan will feel justice was done when their Brazilian goalkeeper Dida comfortably saved compatriot Menegazzo's weak spot-kick.

The home side increased their lead nine minutes after the break.

Rossi could only parry Cristian Brocchi's vicious 30-yard drive and Tomasson reacted quickly to snap up the rebound.

Shevchenko hit the post before Flo netted for the visitors from an acute angle in the closing stages, but Milan held their nerve to secure their fourth consecutive league victory and leapfrog Roma.