US senators called in a letter released Monday for an investigation into whether US companies have provided Internet monitoring and censorship technology to Syria, aiding its crackdown on dissent.

Democratic Senators Bob Casey and Chris Coons as well as Republican Senator Mark Kirk made the appeal in a letter, which was dated Friday, to US Secretary of State Hillary Clinton and Commerce Secretary John Bryson.

US firms should not "provide tools of repression to murderous regimes," they said, citing news reports that Syria has been using technology from from California-based NetApp, Inc. and Blue Coat Systems to track regime foes.

The lawmakers asked Clinton and Bryson to verify the reports, and determine whether the two firms violated a US ban on such exports to Syria and whether the know-how had helped Damascus in its bloody crackdown on protestors.

A top US diplomat, Jeffrey Feltman, last week told Casey in a hearing on Syria that the US Commerce Department was looking into the reports about Blue Coat "because there was no license issued to send this stuff to Syria."

"There were no export licenses issued for this, and the Department of Commerce is investigating it. I would defer to them on the state of the investigation," he added.

A spokesman for the Commerce Department's Bureau of Industry and Security Office, Eugene Cottili, declined to comment, saying "we don't discuss investigations."

But Blue Coat Systems told AFP in late October that Internet filtering equipment sold to Iraq's communications ministry has mysteriously been put to use in Syria, insisting it did not know how the equipment changed hands.

The United States bars selling any such equipment to Syria.

"The evidence points to it being in Syria," a Blue Coat official said, referring to analysis of data logs and computer address numbers from Syria's Internet posted by "hactivists."

"Since we didn't sell it there, we don't know the particulars," said the official, who asked not to be named due to the sensitivity of the matter.

The United Nations estimates that more than 3,500 people, mostly civilians, have been killed in the bloody repression of anti-regime protests that have rocked Syria since mid-March.