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* FTSEurofirst 300 down 0.9 pct, Greece's ATG down 5 pct
* FTSEurofirst still up around 13 pct since start of 2015
* Uncertainty over Greece's debt weighing on markets
* Europe bourses in 2015: link.reuters.com/pap87v
* Asset performance in 2015: link.reuters.com/gap87v
By Sudip Kar-Gupta
LONDON, June 5 (Reuters) - European shares dropped on Friday, weighed down by a slump in the Athens stock market, which slid to one-month lows after Greece delayed a debt payment.
Greece's benchmark Athex General Composite (ATG) index tumbled 5 percent, briefly dipping to its lowest level since June 6, underperforming a 0.9 percent decline on the pan-European FTSEurofirst 300 index.
Switzerland's Syngenta was among the worst performers on the FTSEurofirst, retreating 3.3 percent on worries about regulatory obstacles to its deal with U.S. agrochemicals firm Monsanto.
Greece delayed the payment to the International Monetary Fund, due on Friday, as Prime Minister Alexis Tsipras demanded changes to tough terms from international creditors for aid to stave off default.
Economic stimulus measures from the European Central Bank (ECB), coupled with record low interest rates, have cushioned the impact of Greece on European stock markets overall.
Nevertheless, traders viewed the Greek impasse as an opportunity to sell European stocks, many of which have had a good run so far in 2015, with the FTSEurofirst 300 up 13 percent, although the Greek market is down 5 percent.
"Assurances from Prime Minister Tsipras that Greece will remain in the euro have not exactly soothed investor sentiment today," said London Capital Group's head analyst Brenda Kelly.
The lingering uncertainty over Greece has come against a volatile market backdrop in which yields on German Bunds rose to eight-month highs earlier this week.
European stock markets also stayed in negative territory after U.S. employment figures were released.
U.S. jobs growth accelerated sharply in May and wages picked up, providing signs of economic momentum that bolstered prospects for a September interest rate hike from the U.S. Federal Reserve.
Even though the data pushed the U.S. dollar up against the euro, with a weak euro currency typically benefiting European exporters, traders said worries over Greece meant the region's stock markets remained under pressure.
"The solid U.S. data could lift the European stock markets, but the central focus remains on Greece, which is still unclear and bringing an element of fear to the markets," said ActivTrades chief market analyst Carlo Alberto de Casa.
Today's European research round-up (Additional reporting by Liisa Tuhkanen and Francesco Canepa; Editing by Kevin Liffey)