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June 23 (Reuters) - Carnival Corp CCL.N on Tuesday forecast third-quarter profit below estimates, as lingering impact from higher fuel prices remains a drag on the cruise operator's margins amid geopolitical tensions, sending its shares down about 10% premarket.

Cruise operators, heavily dependent on fuel oil and marine gas oil, had been navigating a tougher environment after the Middle East conflict stoked concerns about prolonged supply disruptions.

Carnival, which is the only major U.S. cruise line that typically does not hedge fuel, said in the quarter it is "overcoming extreme geopolitical headwinds and nearly 30 percent higher fuel costs".

In March, Carnival said it expects impact from higher fuel costs to be more than $500 million.

The company expects quarterly adjusted earnings per share to be about $1.35, compared with analysts' estimates of $1.42, according to data compiled by LSEG.