As organizers of a flotilla seeking to challenge Israel’s naval blockade of the Gaza Strip prepare to embark on their voyage, Israeli officials are mounting an increasingly vocal campaign to discredit the activists, depicting them as planning violence against troops preparing to intercept the ships.

The effort reflects a high level of concern among Israeli officials about the potential impact of the maritime protest. Israeli commandos who boarded a Turkish ship in a similar flotilla 13 months ago encountered resistance and killed nine people, provoking international condemnation that forced Israel to ease its land blockade of Gaza.

On Tuesday, Israeli newspapers were filled with reports from unnamed military officials, charging that sacks of chemicals, including sulfur, had been loaded onto flotilla vessels with the aim of using the materials against Israeli soldiers. The reports, citing military intelligence sources, said that some activists had spoken in preparatory meetings of their desire to “shed the blood” of soldiers and had threatened to kill those who might board their ships.

“Coming to kill,” said a headline in the Maariv newspaper over a photo of one vessel.

Participants in the flotilla — expected to be made up of 10 ships with pro-Palestinian activists from various nations, including a group from the United States with the prominent American author Alice Walker — have insisted that they will react nonviolently to any Israeli attempt to intercept them.

Israel has said that it will not allow the ships to reach Gaza and that it will use force if necessary. Israeli officials say the naval blockade of Gaza is meant to prevent the smuggling of weapons into the territory, which is controlled by the militant Islamic group Hamas.

Dror Feiler, an Israeli-born activist from Sweden and an organizer of the flotilla, dismissed the reports of violent intent in an interview Tuesday on Israeli army radio.

“We have no intention of confronting anyone,” he said. “All our passengers sign a declaration of nonviolence. We are training for nonviolence, to avoid a repetition of what happened last time.

“The state of Israel, with all its army, security services and everything it has, is going against a bunch of 20 nongovernmental organizations,” Feiler added. “Really, it’s ridiculous.”

In Athens, Scandinavian organizers said Tuesday that one of their ships had been sabotaged by divers who cut its propeller shaft. The ship, the Juliano, was docking in the port of Piraeus and is expected to carry Swedish, Norwegian and Greek activists.

The organizers said that the damage can be repaired and that the ship, named for Juliano Mer-Khamis, an Israeli Arab actor and activist who was shot to death in the West Bank, was expected to sail for Gaza toward the end of the week.

An Israeli army spokeswoman declined to comment on the incident.

Efforts to discredit the flotilla have also been made through social media. But a YouTube video promoted by Israeli government offices and purporting to show a gay rights activist whose offer of help was spurned by flotilla planners has turned out to be a hoax.

In the video, a man identifying himself as Marc claims that his offer on behalf of a network of gay rights activists to bring supplies to the flotilla was rejected. He warns that the flotilla organizers were embracing Hamas, one of whose leaders had denounced gays as “a minority of perverts.”

“Be careful who you get in bed with,” Marc says. “You might wake up next to Hamas.”

The video was posted on the private Twitter account of an intern working in the office of Prime Minister Binyamin Netanyahu, and later cited in Twitter feeds from the Israeli government press office and the Israeli Foreign Ministry.

After the Electronic Intifada, a pro-Palestinian Web site, identified the man in the video as Omer Gershon, an Israeli actor from Tel Aviv, the press office apologized on Twitter for promoting a hoax and the Foreign Ministry removed a link to the video on its Twitter feed. A spokesman for Netanyahu said the intern had acted without authorization.