(Updates with comment from Nasdaq spokesman in the third paragraph and with additional details throughout.)

By Jacob Bunge and Kaitlyn Kiernan

Nasdaq OMX Group Inc. (NDAQ) on Thursday halted trading in some contracts for the rest of the day on its largest options exchange due to "system issues," according to notices sent to traders.

Trading in some options contracts with symbols beginning with the letter Z was halted on Nasdaq's PHLX options exchange, while halts were called for certain contracts linked to the stocks of companies such as International Business Machines Corp. (IBM) and Netflix Inc. (NFLX).

"These options will be halted for the remainder of the day while normal trading will continue on NASDAQ OMX PHLX for other symbols. We will continue to communicate to customers when the issue is resolved," said Robert Madden, spokesman for Nasdaq.

The issues began with options symbols beginning with Z at 9:48 a.m. EDT, according to a notice from traders. As of Thursday, PHLX lists 20 stocks beginning with Z, including Zynga Inc. (ZNGA), according to a list posted to the exchange operator's website. Meanwhile, trading was halted in June expiration contracts on seven other names, including IBM, Netflix, Sprint Nextel Corp. (S) and the iShares Silver Trust (SLV) exchange-traded fund.

The options halted on PHLX are still available for trading on some of the industry's other 10 exchanges.

The trading issues "must be minor," according to Joseph Cusick, senior vice president of education at The Charles Schwab Corp. subsidiary optionsXpress. Staff at the online brokerage hasn't heard of any other issues at the exchange, he said.

The PHLX, formerly known as the Philadelphia Stock Exchange, is the largest of the three U.S. options exchanges run by Nasdaq OMX. The exchange executed 20% of all options contracts traded so far this month, according to figures from industry clearinghouse OCC.

Write to Jacob Bunge at jacob.bunge@dowjones.com and Kaitlyn Kiernan at kaitlyn.kiernan@dowjones.com.

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