An international maritime court will rule this month whether to order Russia to release 30 crew members of a Greenpeace ship held since mid-September in a high-profile case brought by The Netherlands.

Russia, as expected, did not attend the near three-hour hearing at the German-based International Tribunal for the Law of the Sea, based in the northern port city of Hamburg.

Its angry response after two Greenpeace activists scaled a state-owned Gazprom oil platform to protest at Russian oil exploration in the Arctic has sparked an international outcry.

Russian coastguards boarded the Dutch-flagged Arctic Sunrise icebreaker on September 19, arresting the 30 crew members, who included activists from 18 countries and two journalists, initially charging them with piracy.

The Netherlands urged the tribunal, established by the United Nations to help settle maritime disputes between states, to take provisional measures, including releasing the crew and allowing the ship to sail.

Liesbeth Lijnzaad, a legal adviser for the Dutch foreign ministry, told the tribunal's judges that Russia had infringed the ship's freedom to sail and violated the crew's human rights.

Expressing regret for the empty Russian seats at the tribunal, she accused Moscow of pursuing legal action for "apparently groundless" reasons. "The disagreement is worsening and spreading," she said.

The tribunal's president Shunji Yanai said a decision on the Dutch demands would be announced on November 22.

"We're looking forward to that date and we're confident that the decision reflects many of the points that have been made," Lijnzaad said in a statement afterwards.

The crew members have been placed in pre-trial detention in Russia until November 24 and several have complained about their conditions, including being kept in isolation, cold cells and lack of adequate food and clothing.

Russian investigators last month reduced the piracy charge, which carries a maximum 15-year prison term, to hooliganism -- the same charge used against the Pussy Riot punk band for a protest performance against Putin.

The Pussy Riot action in a Moscow cathedral in February 2012 landed two band members in prison for two years.

Greenpeace has said the Arctic activists never received official papers formally lifting the piracy charge.

Jail for 'a small yellow banner'?

Russian Prime Minister Dmitry Medvedev said after talks last week with his French counterpart that Russia would not condone attempts to "influence technically complicated and unsafe processes" associated with the extraction of natural resources.

Objects such as oil platforms are essentially "like a powder keg", he said.

Ahead of the hearing, around a dozen Greenpeace activists had rallied in front of the tribunal building.

"As things stand, the Russian authorities propose to jail 30 men and women for two decades because a couple of peaceful protesters tried to hang a small yellow banner from the side of a 500,000-tonne oil platform," Greenpeace International executive director Kumi Naidoo said in a statement.

"In our view, there's every prospect the tribunal will order the release of the Arctic 30, pending the arbitration case that The Netherlands has filed against Russia," he added.

German Chancellor Angela Merkel has voiced concern to Russian President Vladimir Putin over the detentions and French Prime Minister Jean-Marc Ayrault has also called for the release of the French national among the crew.

In Britain, Canada and Argentina, relatives and supporters of the activists including British actor Jude Law have also protested.

Greenpeace has said that 11 Nobel Peace Prize winners have also called on Putin to see that "excessive charges of piracy" laid against 30 activists are dropped.

During the protest in the Barents Sea, Greenpeace hitched two activists to the side of the rig.

The pair tried to scale the platform but eventually slipped into the freezing water and were recovered by the Russian coastguard.

Russian security services seized control of the activists' vessel the next day by descending onto the deck from helicopters in a commando-style raid.