Shurat HaDin -- Israel Law Center -- said it sent a letter to Inmarsat PLC warning the London-based company of the alleged violation.
"We will not tolerate Inmarsat's -- or any corporation's -- profiting from the blood of innocent people," said Nitsana Darshan-Leitner, founder of the center. "Anything short of immediate and decisive action on our part would be akin to acceptance. It is a simple issue of justice: Inmarsat must uphold its legal obligations in compliance with U.S. Treasury regulations and immediately cease its support for Iran."
The U.S. Treasury Department announced earlier this month that sanctions against Iran were being stepped up.
Shurat HaDin noted in its release that it had sued Inmarsat in U.S. District Court in Miami June 2011, alleging the company was supporting ships seeking to breach Israel's naval blockade of Gaza.