UPDATE 3-Oil stumbles as U.S. supply growth offsets OPEC optimism

* OPEC, non-OPEC seen keeping oil cuts until end of 2018

* Rising U.S. crude inventories, output cap prices

* Shanghai crude futures fall 10 pct since Monday's launch

* Goldman says Shanghai initiative "relatively successful" (Adds comment, refreshes prices. Changes dateline from Singapore)

LONDON, March 29 (Reuters) - Brent oil lingered below this week's highs on Thursday, as optimism over OPEC's commitment to controlling its output was partly offset by another rise in U.S. inventories.

The oil price touched $71 a barrel on Tuesday, near its high for the year, but has struggled to gain further traction since then, despite supportive comments from the Organization of the Petroleum Exporting Countries.

June Brent crude futures were down 11 cents at $68.65 a barrel by 0911 GMT, while the May contract, which expires later on Thursday, was down 9 cents at $69.44.

U.S. WTI crude futures were at $64.51 a barrel, up 13 cents from their last close.

Oil has risen by 4 percent since January, on track for its third consecutive quarter of price increases and the longest stretch of quarterly gains since late 2010.

"Right now, oil looks fragile," Petromatrix strategist Olivier Jakob said. "The price action last week was pretty clear. The objective on that move was to take out the highs of 2018, but that's not been done and the price action of the last three days has not been very convincing,"

OPEC, Russia and some other non-OPEC producers started cutting output in January 2017, lifting the price of Brent - the benchmark for most of OPEC's exports - by about a quarter since.

Sources at OPEC said the group and its allies were likely to keep their deal on cutting output for the rest of 2018 when they meet in June.

But rising inventories and production in the United States has capped gains in crude prices. Commercial U.S. stocks rose by 1.6 million barrels in the last week <C-STK-T-EIA> to 429.95 million barrels, while output hit a record 10.43 million bpd, the Energy Information Administration (EIA) said.

This week marked the launch of the Shanghai crude oil futures contract, which has lost about 10 percent since it first opened on Monday. It ended Thursday's session at 409.7 yuan ($65.18) a barrel.

Despite high volatility and scepticism about Shanghai's trading hours and delivery conditions, most analysts expect the contract to establish itself as a third global oil price benchmark next to Brent and WTI.

Goldman Sachs described the start of trading as having been "relatively successful."

"It is the first onshore Chinese commodity contract that allows direct trading by foreign investors and is denominated in RMB (yuan), indirectly promoting the use of the Chinese currency," the bank said.

($1 = 6.2856 Chinese yuan renminbi)

(Additional reporting by Henning Gloystein in SINGAPORE; Editing by xxxxx)