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* Vettel fastest in Friday practice
* Pirelli investigating punctured tyres (Adds drivers seeking clarification, FIA discussions)
By Alan Baldwin
SPA-FRANCORCHAMPS, Belgium, Aug 23 (Reuters) - Formula One drivers sought reassurance from tyre supplier Pirelli on Friday after world champion Sebastian Vettel and Ferrari's Fernando Alonso suffered punctures in second practice at the Belgian Grand Prix.
Red Bull's Vettel was fastest in the dry session but the sight of him nursing his stricken car back to the pits at slow speed with strips of right rear tyre buckling and flailing was more of a talking point.
Pirelli had to strengthen their tyres after a spate of blowouts at the British Grand Prix in June triggered safety fears and threw the sport into crisis.
Alonso had a rear right puncture too, also at around turn 14, at the end of the session and reserved judgement on the causes.
"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." said the Spaniard who was quickest in the morning.
Compatriot Pedro de la Rosa, who heads the Grand Prix Drivers' Association, told Reuters they had asked FIA race director Charlie Whiting for clarification at their regular Friday briefing.
Pirelli motorsport head Paul Hembery assured reporters that Friday's incident was completely different to Silverstone and due to "external sources" - such as debris - rather than anything structural.
"It looks on the Red Bull as though something has been rubbing on the surface and then has just cut through," he said, before talks with Whiting and FIA officials.
"And on the Ferrari there are two quite clear holes through the top of the tread. So we have to go and look at the track later to see what is between turns 13 and 15."
Hembery was still at the track late in the evening and was due to provide an update on Saturday.
Last month's Hungarian Grand Prix, the last race before the summer break, was free of tyre-related incidents but the Hungaroring is one of the slowest circuits while Spa is the second fastest track with heavy loads on the tyres.
Any failure or track issue, with cars racing on full throttle for long stretches, could be catastrophic.
"There is clearly something but I cannot tell you what it is," said Hembery. "It is from external sources.
"It is a worry for the sport because we have to go out and find what it is. We will try our very best to identify it and give us an indication of what is causing it."
Vettel had earlier lapped with a best time of one minute 49.331 seconds with the sun coming out after a damp start to the day in the Ardennes forests where the weather is notoriously capricious.
The championship leader's time looked ominous, 0.059 quicker than second-placed team mate Mark Webber and comfortably clear of Frenchman Romain Grosjean in the Lotus with a time of 1:50.149.
Alonso set the largely unrepresentative pace in the opening session with a lap of 1:55.198 as Formula One drivers shrugged off the long August break and got back up to speed.
The Spaniard, third in the championship after 10 of 19 races, was ahead of the two Force Indias of Paul Di Resta and Adrian Sutil with the top three setting their best times in the final half hour of an incident-free session.
The afternoon was more lively, with Dutchman Giedo van der Garde losing control and crashing his Caterham into the barriers at Stavelot. Hembery said the tyres were not to blame.
The McLarens of Briton Jenson Button and Mexican Sergio Perez showed signs of progress, despite the team backing away from earlier assertions that they could challenge for a victory.
Perez was fourth fastest in the morning and eighth in the afternoon, when the team tried out some developments for 2014.
Vettel, who has a 38-point lead over Lotus's Kimi Raikkonen, had been sixth fastest behind fellow German Nico Rosberg in the Mercedes in the morning.
"This morning was tricky with a few raindrops and the track was slippery; it's a lot colder here than Hungary, but that's Spa and part of this circuit," said Vettel.
"This afternoon we had stable conditions and both of us seemed happy with what we had."
Raikkonen, who cried off sick from media engagements on Thursday after weeks of speculation about his future and talk of moves to Red Bull or Ferrari, was sixth in the afternoon.
Mercedes's Lewis Hamilton, winner in Hungary and increasingly looking like Vettel's biggest rival after three pole positions in succession, had a quiet day and was 15th and 12th respectively for Mercedes.
"The car feels very different here with the low downforce set-up, compared to the last race, and we need to refine a few areas to get the balance sorted," said the Briton. (Editing by John Mehaffey and Ken Ferris)