Israel has stepped up its campaign to stop a flotilla of ships breaching its blockade of Gaza by accusing some of the passengers of intending to harm Israeli soldiers if they board the ships.

An Israeli government official said intelligence agents had discovered that extremists aimed to infiltrate the peace activists travelling on the 10-ship flotilla. "On the flotilla, there's an unofficial division of labour. There are activists, writers and politicians who say that they are not aware of anyone with bad motives. We believe them but the people that we are concerned about are avoiding the television cameras like the plague.

"We have put intelligence assets into operation so that we know what kind of people are on the flotilla. We believe there are people, and we don't know how many, who will mix in with the peaceful activists and will use violence directly against our soldiers," the official said.

The organisers of the flotilla say that all participants have undergone compulsory training in non-violent resistance and have pledged not to use violence. One organiser, Huwaida Arraf, said she was mystified by Israel's claims. "We don't know where they are getting this from. We are very open about who we are and what we are doing."

Arraf said the flotilla had faced other problems but they hoped to overcome them. One ship, the Juliano, is being repaired after a suspected underwater sabotage attack. The ship's propeller shaft was partially sawed while it was docked at the Greek port of Piraeus. Another ship, the Audacity of Hope, has been banned from leaving port until Greek authorities agree it is seaworthy.

"We can't say for certain that Israel was responsible for the sabotage but no one else wants to stop us sailing," said Arraf.

The flotilla was due to sail this week but it is expected to delayed until next week.

The latest activity against the flotilla follows two incidents which have embarrassed the Israeli government. In one, the director of the government press office in Jerusalem threatened journalists travelling with the flotilla that they risked having their equipment confiscated, being banned from Israel for 10 years and other sanctions.

Binyamin Netanyahu, the Israeli prime minister, was quick to disassociate himself from the threat and said journalists would not suffer any sanctions for travelling with the flotilla.

In another incident, government staff circulated a video purporting to be made by a gay rights activist whose offer to join the flotilla had been rejected. The activist said he was rejected because the organisers were associated with Hamas which was homophobic and antipathetic to human rights.

When the video was exposed as a hoax, the Prime Minister's office said the video was promoted by an intern, Guy Seemann. "Mr Seemann is a 25-year-old who is interning in our office. His tweet was a mistake on his part. It was done without authorisation and without approval. His mistake has been pointed out to him."