A Russian court on Thursday began considering whether to prolong the detention of 30 Greenpeace activists suspected of piracy after they held a high seas protest on an Arctic oil platform.

The Lenin district court in the northern city of Murmansk on Thursday morning opened a hearing into the activists, including 26 foreigners, who launched a protest on an oil rig from Greenpeace's Arctic Sunrise ship.

Russian investigators said they would ask the court to rule that all the activists be kept behind bars as the piracy case is investigated further. They have been held in detention in Murmansk since late Tuesday.

The hearing began shortly after 11:00 am in Murmansk (0700 GMT), Greenpeace Russia wrote on Twitter.

"The hearing will be open... They will start with citizens of France, Canada and Russia," Greenpeace said.

It said that activists were brought into the court building in handcuffs.

"Today the investigation intends to ask for the Lenin district court of Murmansk to hold all the 30 suspects in custody," the Investigative Committee, which probes major crime cases in Russia, said on its website.

The Russian authorities by Thursday evening must either charge the activists, release them or apply for their detention to be extended without them being charged, Greenpeace spokesman Aaron Gray-Block said in an e-mailed statement.

He said the activists had all been interviewed with a lawyer and some with a diplomat from their country also present.

The Investigative Committee said that all the activists, who come from 18 different countries including Britain and the United States had cited their right to remain silent under the Russian Constitution.

Russian border guards took control of the Greenpeace ship and locked up the activists after they last week attempted to scale state energy giant Gazprom's Prirazlomnaya oil platform in protest at oil exploration in the Barents Sea.

The border guards, who had seized the vessel after descending on it on ropes from helicopters, then towed the ship to Murmansk, where the activists were held for questioning.

Russia has opened a case into piracy, although President Vladimir Putin took a milder stance on Wednesday, telling an international Arctic forum that "of course they are not pirates."

This has raised hopes that even if they are eventually charged, it will be under a less grave article than piracy.

But Putin said the activists had broken international law by approaching dangerously close to the oil platform.

Greenpeace denies that the activists committed piracy, saying that this only applies to ships and that they held a peaceful protest.