Oslo (AFP) - Donald Trump, Greek islanders helping desperate migrants, Angela Merkel and the pope -- some may seem more likely than others but all are understood to be in the running for the 2016 Nobel Peace Prize.

Nominations for the award, won last year by four Tunisian groups that led the country's transition to democracy, must be posted to Norway by February 1 at the latest.

And as usual, the names of the nominees are a tightly-guarded secret -- or at least most of them are. Those who nominate candidates can reveal the name of the person they've proposed.

An online petition had on Monday picked up some 630,000 signatures calling for the Nobel to go the residents of Greek islands on the front line of Europe's migrant crisis, who have come to the aid of refugees turning up on their shores after perilous sea journeys from neighbouring Turkey.

The nomination, proposed by a group of university professors, faces a size problem: the prize can be shared by a maximum of three laureates.

Greek scientists have resolved the problem by putting three names forward: an octogenarian, a fisherman from Lesbos and the Hollywood actress-activist Susan Sarandon, who was the first high-profile celebrity to visit the island to raise awareness about the issue.

South African Archbishop Desmond Tutu, who won the 1984 Peace Prize, has backed three nominations, including one involving Greece's Good Samaritans called the Aegean Solidarity Movement.

"Just imagine 900,000 visitors in desperate need arriving at the door of your reasonably modest establishment. Hungry, exhausted and in a state of acute emotional distress... They don't speak the same language as you or ascribe to the same cultural or religious beliefs. What do you do? You open the door. Incredible!" Tutu wrote on his foundation's website.

The two other nominations receiving his blessing were the Nuclear Age Peace Foundation, and a trio that includes Pope Francis, hailed by the Anglican archbishop for "consciousness about the ecological necessity to curb human consumptiveness and greed."

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- Prize to a 'traitor'? -

In an entirely different field, US Republican presidential hopeful Donald Trump has also been nominated for the prize, according to Nobel watcher Kristian Berg Harpviken, the director of the Peace Research Institute of Oslo.

According to a copy of the nomination letter Harpviken said he had received, brash tycoon Trump -- who has attracted international condemnation by calling for a ban on Muslims entering the United States -- deserves the prize for "his vigorous peace through strength ideology, used as a threat weapon of deterrence against radical Islam, ISIS, nuclear Iran and Communist China".

Thousands of people around the world are allowed to make nominations for the Peace Prize, including members of parliament and government ministers, former laureates and some university professors.

The Nobel Institute accepts all valid nominations, so having one's name on the list is not to be taken as a sign of approval.

The five members of the panel that selects the laureate are also allowed to put forward their own nominations when they hold their first meeting on February 29.

Harpviken said he believed Edward Snowden, the American who exposed mass surveillance by the US National Security Agency, could be a winner this year.

"Snowden's leaks led to a good number of reforms in US practice and US legislation, which make it harder to still argue that he is a traitor to his country," Harpviken told AFP.

After a breakthrough on the Iran nuclear standoff, negotiators Ernest Moniz of the US and Ali Akbar Salehi of Iran are also among Harpviken's favourites, as well as Colombian peace negotiators President Juan Manuel Santos and rebel leader Timoleon Jimenez.

German Chancellor Angela Merkel, who was one of the favourites last year when the prize went to Tunisia's National Dialogue Quartet, is also in the running again this year.

The same goes for Congolese gynaecologist Denis Mukwege, who has spent a quarter of a century treating thousands of women brutalised by rape and sexual violence in war-torn eastern Democratic Republic of Congo.

Nadia Murad, a Yazidi abducted by IS fighters in August 2014 from her village in Iraq and held for three months as a sex slave is also in the running.

Finally, 118 Italian MPs have nominated the Afghan Cycling Federation women's team, hailing the bicycle as environmental, economic and democratic.

The winner of the 2016 Nobel Peace Prize will be announced in early October.