The NGO representing most of the activists detained aboard a Gaza-bound aid boat that was intercepted by Israeli forces said that two of the campaigners had been placed in solitary confinement in Israel.
“Israeli authorities transferred two of the volunteers – the Brazilian volunteer Thiago Avila and the French-Palestinian European Parliament member Rima Hassan – to separate prison facilities, away from the others, and placed them in solitary confinement,” Israeli human rights group Adalah said in a statement.
When asked for comment, Israel’s prison authority referred AFP to the foreign ministry, which claimed it was checking the reports.

The Freedom Flotilla Coalition said in a statement on Instagram that Mr Avila is “on his third day of hunger and water strike denouncing Israel’s impunity”.
The group, who organised the boat’s attempt to bring humanitarian aid to Gaza, said Adalah’s lawyer informed them that Israeli authorities have threatened to put Mr Avila in a “dark, small and breathless and contactless cell” for seven days.
It also accused Israel of holding the activists as hostages.
Activists ‘capitalising’ on conflict for attention – France PM
Meanwhile, France’s Prime Minister François Bayrou accused the French activists of capitalising on the Israeli-Palestinian conflict for political attention.
Ms Hassan is an MEP from the France Unbowed (LFI) party and is among four French activists still detained in Israel.
Israeli forces intercepted the Madleen and its 12 crew members in international waters off the besieged Palestinian territory on Monday.
Another four, who are not French, were also taken into custody.
The remaining four, including two French citizens and Swedish campaigner Greta Thunberg, agreed to be deported immediately after being banned from Israel for 100 years.
LFI leader in parliament Mathilde Panot accused the prime minister of failing to condemn Israel’s actions.

“These activists obtained the effect they wanted, but it’s a form of instrumentalisation to which we should not lend ourselves,” Mr Bayrou responded in the National Assembly.
It’s “through diplomatic action, and efforts to bring together several states to pressure the Israeli government, that we can obtain the only possible solution” to the conflict, he added.
France and Saudi Arabia are co-hosting a UN meeting later this month in New York on steps towards recognising a Palestinian state and reaching a so-called two-state solution to the conflict.
France’s Foreign Minister Jean-Noël Barrot told parliament the priority in Gaza should be “an immediate ceasefire, the release of all hostages held by Hamas, as well as immediate, unimpeded and massive humanitarian aid access to abridge the suffering of civilian populations”.
“In no way whatsoever do the gesticulations of Ms Rima Hassan, her instrumentalisation of the suffering of Gazans, help to achieve these goals,” he added.
He said the French consul had visited all four French activists in Israeli detention.
The Israeli ambassador in Paris earlier said the Israeli authorities aimed to put them onto a plane back home “as soon as possible”.
Israel is facing mounting pressure to allow more humanitarian aid into Gaza, whose entire population the United Nations has warned is at risk of famine.