DUBAI - South Africa were docked five penalty runs for ball-tampering on the third day of the second Test against Pakistan in Dubai on Friday.

Television replays showed South African fielder Faf du Plessis rubbing the ball on his trousers, which had a zip on them, in an apparent attempt to tamper with the ball.

Fast bowler Vernon Philander also caught by the camera while scratching the ball.

Umpires Rod Tucker of Australia and England's Ian Gould called South African captain Graeme Smith, changed the ball and added five runs to Pakistan's score.

Pakistan were struggling at 62 for three with Dale Steyn about to start the 31st over when the umpires took the decision.

The ball was then replaced from a box carried on to the pitch by fourth umpire Shozab Raza after which Gould signalled a five-run penalty, effectively accusing the top-ranked test side of unnaturally trying to alter the state of the ball.

"As per 42.1 of the ICC playing conditions, the umpires replaced the ball and fined South Africa five penalty runs for ball-tampering," said an International Cricket Council (ICC) spokesman.

A decision on the incident is expected early Saturday (today) although the ICC refused to confirm details.

The level two offence of changing the condition of the ball carries a fine of 50 to 100 per cent of a player's match fee or a ban of one Test, two one-dayers or two Twenty20 matches, whichever comes first for South Africa.

South African vice-captain AB De Villiers denied the charges. "Honestly, we're not the team that scratches the ball," said De Villiers.

"We play in a fair manner. Obviously you want to swing the ball as much as you can and try to get it to reverse, putting sweat on the one side and stuff like that. We don't cheat, it's as simple as that.

"I know 'Faffy' very well, he's the last man to try anything like that, it is part of his responsibility to shine the ball and to get it to swing and to look after it. It's not an easy job, I thought he did it very well."

Asked did the incident shock the Proteas, De Villiers replied: "Honestly, I don't know what happened there, I don't even know from where the message came, obviously very surprised.

"There was no talk about it, no warnings, nothing, it was just out of nowhere, I don't know the facts, I have said what I had to say about Faf and it's pretty much about it."

Neither Smith nor Du Plessis spoke to the media after the match and Cricket South Africa did not even mention the incident in two separate summaries of the day's play sent to the media on Friday.

Pakistan are the only other team to have been penalised with five penalty runs, during the infamous Oval Test against England in 2006. Former pacemen Waqar Younis, Shoaib Akhtar, all-rounder Azhar Mahmood and leg-spinner Shahid Afridi have all been banned for tampering in the past. India's Sachin Tendulkar and England's Mike Atherton were also suspended and fined for tampering.

Bowler John Lever was accused by India of using vaseline to get unnatural shine on the ball as England dominated the away series in 1974.

New Zealand worked on the ball with a bottle top during the third test against Pakistan in Faisalabad in 1990, said wicketkeeper-batsman Adam Parore several years later.

England bowlers Stuart Broad and James Anderson were accused of deliberately standing on the ball with their spikes during a test against South Africa in 2010.

Australia seamer Peter Siddle was cleared after being accused by Sri Lanka of attempting to raise the seam during a test last year.

Former England captain Bob Willis accused his fellow countrymen of ball-tampering during this year's Champions Trophy but the players vehemently denied the suggestion.