A senior United Nations envoy on Sunday condemned a wave of attacks by Israeli settlers on Palestinian olive groves and called on Israel to punish the perpetrators. "I am alarmed at recent reports that Israeli settlers in the West Bank have repeatedly attacked Palestinian farmers and destroyed hundreds of their olive trees at the height of the harvest season," said Robert Serry, UN special coordinator for the Middle East peace process. "These acts are reprehensible and I call on the government of Israel to bring those responsible to justice," he said in a statement. Israeli rights group Yesh Din said it had logged 35 attacks targeting olive trees, grapevines and other fruit trees between September 2011 and July 2012, but in 99 percent of cases police had closed their probes for lack of evidence. "The phenomenon of tree vandalism continues even more forcefully this year and targets the property and livelihood of many Palestinian families," said Yesh Din. "The police's failure to enforce the law and protect the Palestinians' property encourages this phenomenon because the criminals who go unpunished are not deterred from repeating their actions." Israeli police challenged Yesh Din's findings. "They are inaccurate and out of date," spokesman Micky Rosenfeld told AFP. "Police have stepped up activity, particularly over the past few weeks, in dealing with the incidents which have been taking place." "There was an undercover police operation which took place just under two weeks ago where three police officers dressed up as Palestinian shepherds were attacked (by settlers)," he said, adding that the officers were specifically watching out for attacks on Palestinian farmers at the time. "Israeli police have arrested four people who were directly connected in that attack." On Saturday, dozens of olive trees were set ablaze in the West Bank village of Qaryut village, south of Nablus, in an attack villagers and Palestinian security officials blamed on settlers. Yesh Din quoted Qaryut farmer Said Ahmad Jaber as saying that just a few days earlier, on Tuesday, he had 120 olive trees cut down. Rosenfeld said an investigation into those attacks was under way but no arrests had so far been made. A coalition of Israeli rights groups said hundreds of trees had been damaged or had their crop stolen since October 7. "The past week was unusual in terms of the extent of the theft and destruction of Palestinian olive groves, especially those near settlements and outposts known as trouble spots," they said. "According to early estimations, over 450 Palestinian trees have been damaged this week." The threat faced by Palestinian olive farmers during the harvest, which began last week, was raised by senior Palestinian official Hanan Ashrawi in a letter to diplomats. "Palestinians across the country have begun the annual olive harvest this month and once again they are being attacked by violent Israeli settlers," she wrote in the letter, a copy of which was seen by AFP. "The Palestinian people request that your mission to our country send observers to all major at-risk olive picking areas to document Israeli settler and military abuses. In addition we believe your presence will be a deterrent to further violence." Figures released last year by aid group Oxfam indicated there are about 9.5 million olive trees in the West Bank, where the crop is a vital source of revenue for Palestinian farmers. In a good year, the olive harvest contributes around $100 million (77 million euros) in income to some of the poorest Palestinian communities.