UPDATE 2-United Air lowers 3rd-qtr forecast in wake of Harvey, N. Korea

(Adds background on Harvey, details from transportation conference, stock price)

Sept 6 (Reuters) - United Continental Holdings Inc on Wednesday said third-quarter results will be sharply lower than forecast, as Hurricane Harvey and geopolitical tensions between the United States and North Korea weigh on the airline's performance.

Shares fell 4.2 percent to $58.51.

Harvey was the largest operational disruption in the Chicago-based carrier's history, it said, and will cut third-quarter passenger unit revenue between 3 percent and 5 percent, Chief Financial Officer Andrew Levy said at the Cowen and Co Global Transportation Conference.

United forecast that the closely watched performance metric would be flat in the third quarter.

Levy also blamed the standoff between the United States and North Korea, including North Korea's threat to fire missiles near U.S. territory Guam, for some depressed performance.

"The closure of our Houston hub because of Harvey is estimated to have a significant negative impact on our third-quarter financial results. Jet fuel prices have increased, even more so after Harvey, and our revenue is below our initial forecast due to a variety of reasons," Levy said in a Wednesday letter to United employees.

Harvey has weighed on the U.S. travel industry after it destroyed homes and businesses in Texas, especially in Houston, flooding entire neighborhoods and grinding travel in and out of the region to a halt. An estimated 60 people lost their lives.

Operations at United's second largest hub, George Bush Intercontinental Airport in Houston, were suspended for over four days, forcing over 7,400 flights cancellations.

United now projects flight capacity will be slightly lower, between 3 percent and 3.5 percent, from its earlier forecast of 4 percent.

"Look, we lost a lot of revenue," Levy said at the conference, adding that the carrier will only be able to recoup a small amount of the losses in other markets.

"We had to just walk away from a large amount of revenue during what is an important period of the year for us."

The U.S. airline industry, still recovering from Harvey, is bracing for more disruptions as Hurricane Irma approaches Puerto Rico and the U.S. mainland.

By Wednesday, numerous flights and cruises were canceled, but as Irma's path becomes clearer in the next few days, more cancellations are likely, costing the industry millions of dollars more on top of already soaring costs. (Reporting by Alana Wise in New York; Editing by Jeffrey Benkoe)