Some of the 28 Greenpeace activists and two journalists being detained in Russia while awaiting piracy charges are being kept in solitary confinement for 23 hours a day, while others are held in "extremely cold" cells, according to the head of Greenpeace.

Kumi Naidoo, Greenpeace International`s executive director, told the Guardian that the crew had been split up into several prisons across the port city of Murmansk in north-west Russia, which is in the Arctic Circle. Three of the group have been sent to a prison 150km away.

Naidoo said the organisation would have to "take into account" the way the protesters had been treated, but he added that Greenpeace had not been silenced by intimidation in the past and would continue to show "leadership" on the Arctic issue.

Over the weekend, Vigils were held around the world to free the "Arctic 30", with celebrities including actor Jude Law and musician Damon Albarn joining an estimated 800 people who gathered outside the Russian embassy in London.The activists, who come from a total of 18 countries and include six Britons, were detained after the Russian coastguard seized their vessel following their protest in September against a Gazprom-owned oil platform in the Arctic, Russia`s first offshore drilling in the Barents Sea.

Naidoo said: "Some of the colleagues are in cells which are pretty warm and reasonably OK to be in … whereas some are in cells that are extremely cold. This is the Arctic we are talking about."

Although several of the activists were being held in cells where they could interact with one another, Naidoo said that "some are held in solitary where they are in the cell for 23 hours and they have one hour for exercise".

Naidoo said he did not think splitting up the campaigners had been done maliciously and that he was grateful they had all met with consular representatives.

One of the Britons being held, Alexandra Harris, who has been named along with videographer Kieron Bryan, Philip Ball, Anthony Perrett, Frank Hewetson and Iain Rogers, became ill earlier this month and was taken to a hospital for examination but did not need treatment. Naidoo said that arrangements had been made for her medication. The other journalist is a Russian photographer, Denis Sinyakov.The Greenpeace activists are being held for what the Russian courts have described as pretrial detention for two months, before facing piracy charges which carry a potential prison sentence of 15 years. "Detention without trial is recognised as being unjust, unfair, and unacceptable," Naidoo said.

The Russian president, Vladimir Putin, recently defended the actions of the coastguard, saying they could not have known who the Greenpeace activists approaching the oil rig were, but said: "I don`t know the details of what went on, but it`s completely obvious they aren`t pirates."

Naidoo said that the strong response by the Russian authorities, which allowed a similar operation by Greenpeace in 2012 without arrests, would not lead him to rule out future actions in Russia.

"The events that have occurred we will have to take into account. We will have to think about what sort of actions we take in the future. As an organisation, we have never been silenced by repression from governments … If we are to save the Arctic, prevent catastrophic climate change, and resisting the inaction on the part of governments who should be taking leadership, we will do that," he said.

Law@guardian