WASHINGTON — An Israeli human rights group is threatening legal action against Inmarsat if the global satellite communications firm continues to support ships seeking to breach Israel’s naval blockade of Gaza later this month.

As the sole satellite provider for the internationally mandated Global Maritime Distress and Safety System (GMDSS), Inmarsat says it has become an unwitting player in political battles raging over Israel’s protracted siege of the Hamas-controlled coastal strip.

But Tel Aviv-based Shurat HaDin insists Inmarsat will face severe criminal and civil penalties if it does not cut off GMDSS service to ships providing material support for Hamas, designated by the United States, Britain and the European Union as a terrorist organization.

In June 5 letters to corporate offices in London and Washington, the group warned that under U.S. law, Inmarsat and its senior executives would “be open to charges of aiding and abetting terrorism if it provides services to the Gaza-bound ships.”

The group noted that Inmarsat provided maritime communications support to the Turkish-flagged Mavi Marmara and two other ships that defied Israel’s blockade of the Gaza Strip in May 2010. Nine activists of the Ankara-based IHH Humanitarian Relief Foundation were killed after violent confrontations with Israel Navy commandos dispatched to divert the ship to Israeli ports.

As the Mavi Marmara and other IHH ships prepare for yet another blockade-busting confrontation with fortified and increasingly resolute Israeli forces, Shurat HaDin insists Inmarsat is uniquely positioned to prevent or, alternatively, precipitate more violence.

“We don’t want bloodshed on the sea again,” said Nitsana Darshan-Leitner, an attorney and founder of the Tel Aviv-based group. “Ships like the Mavi Marmara rely upon Inmarsat’s network to communicate with ports, naval authorities and other vessels and cannot sail without it.”

She added that activists in the private legal sector could help governments prevent terrorism. “The law is very strict when it comes to aiding and abetting terrorism or politically motivated violence and imposes severe criminal and civil liability,” she said.

Inmarsat spokesman John Warehand refuted Shurat HaDin claims that the company could be held liable for supporting terrorism, since the Mavi Marmara and GMDSS-equipped vessels are neither owned nor operated by Hamas or any other terrorist organization.

“We obviously always seek to comply with all international laws and all relevant sanctions legislation … [But] in the absence of a proven link, we see no obvious reasons why we can’t fulfill contractual obligations,” Warehand said.

In a June 9 interview, Warehand said that with specific regard to GMDSS services, the firm operates as a wholesale organization, with no direct ties with ship owners or operators. He noted that under 1999 amendments to the Safety of Life at Sea convention, each ship above a certain weight is required by the International Maritime Organization to install GMDSS.

“We’re the only satellite provider of that system, but we’re not the regulator. But even so, as far as we are aware, IHH — the retail consumer, if you will — is not on any list,” Warehand said.

According to a 2010 Congressional Research Service report, IHH has charitable and other associations with Hamas, al-Qaida and other suspected terrorist groups, but is not on the U.S. State Department’s terrorist list.

Israel banned the organization in 2008 from operating within its borders, and added IHH to its terrorist list in May 2010, following weapons and documents seized from the Mavi Marmara.

Alan Baker, director of the Jerusalem-based Institute for Contemporary Affairs, acknowledged that Gaza-bound flotillas organized by IHH and other groups are primarily public relations vehicles for opposing the blockade rather than terrorist expeditions. Nevertheless, he insisted Israel has the right under international law to enforce its blockade and, if necessary, to use force in response to violent forms of protest.

In a June 5 position paper that closely reflects official policy, Baker, a former Israeli Foreign Ministry legal adviser and ambassador to Canada wrote: “It is internationally accepted that any attempt to breach such a blockade by civilian or other vessels may be prevented by Israeli naval patrols, and the vessels involved may be stopped, searched and sent to an Israeli port in order to check their cargos … Furthermore any vessel refusing to respond to the demands of the naval forces may be stopped forcefully.”