Singapore has long courted international banks and companies in its drive to become a financial hub. But it is also wooing a very different sector: non-profit groups including campaign charities.

Singapore has attracted more than 130 "international non-profit organisations" such as the charities Mercy Relief and World Vision International, and the International Air Transport Association, an aviation lobby group.

This is triple the number of regional or global non-profits that were based in Singapore when a campaign to lure them with tax breaks and other incentives started in 2007.

The government says it is keen to broaden Singapore's international appeal and tap into a growing environmental and charitable consciousness among its wealthy populace.

But the welcome does not extend to every campaigning organisation that has attempted to set up shop in Singapore, whose no-nonsense government is routinely accused by foreign and local activists of squelching dissent.

Those who have made it in include the World Wide Fund for Nature (WWF), one of a clutch of non-profits housed in a dedicated three-building complex called the Tanglin International Centre.

Elaine Tan, chief executive of the Singapore branch of WWF, said the group appreciates the generous allocation of office space in the complex, a luxury hard to find in Singapore's costly business districts.

"The greenery and occasional horn bills and monitor lizards were a bonus! It also offers us a lot more room for future growth," she said.

Tan said companies were increasingly incorporating sustainability and other environmental measures into their business practices.

"So it makes a lot of sense for Singapore to welcome NGOs such as WWF that have a long history of success helping companies reduce their impact on the environment," she said.

The most recent addition is the global headquarters of Earth Hour, which coordinates a one-hour period every year when cities shut off their lights to show support for action against climate change.

Earth Hour, which is moving to Singapore from Sydney, plans its next annual lights-off event on March 31.

"We wanted to bring some of the kind of the organisational skills and talent we see in Singapore to bear on the operation, on how we run it," Earth Hour co-founder and executive director Andy Ridley said last month.

Conservation groups Birdlife International and Flora and Fauna International are also scheduled to open offices in Singapore.

"There are several tangible and intangible benefits that we think (international non-profits) bring to Singapore as they go about their work," Quek Swee Kuan, assistant managing director of the government's Economic Development Board, told AFP.

The groups offer employment and training opportunities for Singaporeans in specific fields like ecology, he said.

This is on top of "intangible" benefits such as providing international volunteer opportunities for "the increasingly passionate and socially conscious people in Singapore", Quek said.

But more radical groups such as Greenpeace, which has a long history of high-profile environmental activism, have so far been shut out.

Greenpeace Southeast Asia director Von Hernandez told AFP he had tried to get the group registered as a company in Singapore in 2010 with the intention of applying for charity status, but it was rejected by authorities.

"No exact reason was given for the disapproval of our application, though I suspect it has to do with our strong position against nuclear energy and the government's fear that we might agitate local citizens if ever the country's plans to develop nuclear energy would advance in the future," he said.

When asked about the absence of groups like Greenpeace, the EDB's Quek said approval for foreign non-profits to be registered as legal entities "is subject to the relevant regulatory framework".

That framework "reflects the social norms and legal structure of Singapore," he said.

Justin Harper, a markets strategist for IG Markets Singapore, said "nothing too controversial, no one too risky" would be allowed into Singapore.

"I think this is just diplomatic moves to show its softer side," he told AFP, noting its "obsession" with becoming a major player in world finance.