A U.S. company that makes Internet-blocking gear acknowledges that Syria has been using at least 13 of its devices to censor Web activity there—an admission that comes as the Syrian government cracks down on its citizens and silences their online activities.

Blue Coat Systems Inc. of Sunnyvale, Calif., says it shipped the Internet "filtering" devices to Dubai late last year, believing they were destined for a department of the Iraqi government. However, the devices—which can block websites or record when people visit them—made their way to Syria, a country subject to strict U.S. trade embargoes.

Blue Coat told The Wall Street Journal the appliances were transmitting automatic status messages back to the company as the devices censored the Syrian Web. Blue Coat says it doesn't monitor where such "heartbeat" messages originate from. Computer code reviewed by the Journal indicates that Syrians were also using other Blue Coat products, raising questions about how the tools came to be used this way and whether Blue Coat has violated the trade embargo.

As Arab Spring political uprisings have swept the region this year, Bashar al-Assad, whose family has ruled Syria for more than four decades, has overseen some of the bloodiest crackdowns on protesters. On Friday, Syrian troops opened fire on protesters, leading to fresh reports of deaths. According to the U.N., more than 3,000 civilians have been killed in Syria since the start of protests.

Blue Coat executives say they don't know how the devices got to Syria. The company says it alerted U.S. authorities in recent days to the "improper transfer" and is cooperating with government inquiries.