Trading resumed on Thursday afternoon on nearly 10,000 over-the-counter equities after a 3-1/2-hour halt on all OTC Markets Group platforms triggered by a connectivity issue.

The Financial Industry Regulatory Authority halted trading in all OTC Equity Securities at 11:25 a.m. "to protect investors and ensure a fair and orderly marketplace," OTC Bulletin Board said on its website. Trading resumed at 3 p.m.

A connectivity issue was behind the halt, said an OTC Markets Group spokeswoman.

Trading in Nasdaq- and New York Stock exchange-listed securities was not affected.

Issues halted included about 1,400 American Depositary Receipts and 1,400 foreign ordinaries. Roughly $8 of every $10 traded in OTC exchanges are in ADRs and ordinary shares of international exchange-listed companies, according to data from OTC Markets' website.

Many securities on the over-the-counter market are illiquid, trading infrequently, with prices usually less than $1.

A raft of technical glitches has affected U.S. equity markets in recent years. In August, there was a three-hour halt in all Nasdaq-listed stocks and an issue at CBOE that suspended options trading in major U.S. options contracts that same month. In 2012, there was a trading error that cost Knight Capital Group $461 million and led to the sale of the firm, now part of KCG Holdings.