European shares sag as Greek stocks' losing streak worsens

* Greece concerns, U.S. rate hike prospects dampen sentiment

* Athens' ATG market falls 3.2 pct

* Telecom Italia up on report Vivendi to increase stake

* Rise in oil price lifts energy stocks

* Europe bourses in 2015: http://link.reuters.com/pap87v

* Asset performance in 2015: http://link.reuters.com/gap87v

By Sudip Kar-Gupta

LONDON, June 17 (Reuters) - European stock markets lost ground on Wednesday as persistent concerns over Greece's debt problems offset gains in Telecom Italia and shares in energy companies.

Traders were also reluctant to add to equity positions before a U.S. Federal Reserve meeting, with many investors expecting the Fed to signal that U.S. interest rates will be raised in September.

Athens' benchmark ATG equity index fell 3.2 percent, underperforming a 0.5 percent decline on the pan-European FTSEurofirst 300 index. Germany's DAX fell 0.6 percent, while France's CAC declined 1 percent.

Car (HKSE: 0699.HK - news) parts maker Valeo (Paris: FR0000130338 - news) dropped 5.4 percent after Credit Suisse (NYSE: CS - news) cut its price target on the stock, while Danish medical equipment maker Coloplast (Xetra: CBHD.DE - news) retreated 3.9 percent after cutting forecasts.

But Telecom Italia outperformed, rising 0.5 percent after sources told Reuters that Vivendi was planning to increase its stake in the firm.

A rise in Brent crude oil prices also boosted energy stocks .

Nevertheless, some traders said they would use any bounce in share prices as a cue to sell, given the concerns over Greece's debt deadlock, which has caused Athens' ATG market to fall for four consecutive sessions.

"We're looking to sell into strength here," Logic Investments director Darren Easton said.

ATHENS ANGST

Easton said the DAX could fall 1,000-2,000 points if no deal was reached over the country's debts and Greece was forced to leave the euro zone.

Athens' ATG index has fallen about 18 percent since the start of 2015, underperforming an 11-percent advance on the FTSEurofirst.

Stimulus measures from the European Central Bank have cushioned blows from Greece on other European economies.

Some investors said any Greek debt default may have a limited impact on the broader market as Greece represents only a small amount of the European economy.

They also highlighted the ongoing monetary support from the ECB as helping to offset the impact from a 'Grexit', namely Greece exiting the euro zone.

"Any sell-off triggered by a Grexit scenario, we would consider a buying opportunity," J.P. Morgan Asset Management global market strategist, Vincent Juvyns, said.

Today's European research round-up (Additional reporting by Atul Prakash and Alexandre Boksenbaum-Granier; Editing by Catherine Evans)