Two deflations and several cuts in the surface of tyres have raised concerns that Pirelli may face another difficult weekend at the Belgian Grand Prix.

Drivers, led by Mark Webber and Jenson Button, have expressed their concern over the safety of the rubber provided for the quick Spa-Francorchamps circuit, despite the revised Pirellis having returned to something close to last year's tyre for the same event in the wake of the chaos seen at Silverstone in late June.

Both Sebastian Vettel and, less visibly, Fernando Alonso suffered punctures during Friday practice in the Ardennes, with several other teams reporting cuts in the surface of their tyres, prompting several drivers to demand answers and guarantees that the rubber was up to the task.

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Pirelli's Paul Hembery suggested that the problems were being caused by debris on the circuit but, with similar cuts having been noted at last year's race, Webber insisted that the explanation did not go far enough.

"We need answers and 'debris' is not the answer," the Australian claimed.

Hembery confirmed that both deflations appeared to have occurred at the same point on the circuit, and that Pirelli was due to despatch members of its staff to the section between turns 13-15 to see if they could uncover a possible cause. Caterham's Giedo van der Garde also crashed out in the same section, although the cause of his accident has yet to be determined.

"It will be important to have a good understanding of how the tyres are behaving in the dry, as well as finding out what happened to the Caterham, to Vettel and also to me on my in-lap," Alonso claimed, "I don't think it's a similar problem to what we saw in Silverstone, maybe more of a random set of circumstances, but, all the same, it needs careful analysis."

Hembery agreed that, in his opinion, the situation was very different to what Pirelli faced at Silverstone, where the race was blighted by three high-speed blow-outs, but conceded that it could not be ignored, particularly as the drivers will not have dropped their threat to boycott any event where they felt their safety was compromised by the tyres.

"With the Red Bull, it looks as though something may have been caught between the tyre and the bodywork, as there are signs of rubbing on the surface," Hembery estimated, "The Ferrari, meanwhile, has two clear holes through the top of the tread, and there are small surface cuts on several other tyres, so there is clearly something amiss. From initial analysis, it appears that both were caused by an outside factor, probably debris on the track.

"It's a worry for the sport because we have to go and find out what it is."