The United Nations on Monday launched a week-long global environment conference aimed at tackling challenges from poaching to marine pollution and boosting the "green economy".

The meeting in Nairobi, the first ever United Nations Environment Assembly (UNEA), comes amid tight security in the Kenyan capital, after a series of warnings of the threat of attack by Somalia's Al-Qaeda-linked Shebab.

The gathering hopes to "influence policy action on environmental themes, ranging from sustainable consumption and production, and financing the green economy, to the illegal trade in wildlife, and environmental rule of law," organisers said.

Key topics discussed Monday include the issue of plastic contamination, which threatens marine life, tourism, and fisheries on vast scale.

Other issues on the agenda include the impacts of excess nitrogen and marine aquaculture, as well as dangers posed by air pollution.

The five-day event run by the UN Environment Programme, headquartered in Nairobi, includes "over 1,200 participants from government, business and civil society, as well as high-level delegations from over 160 UN member and observer states," UNEP said.

UNEA was set up after suggestions made at the UN Conference on Sustainable Development in Rio de Janeiro two years ago.

On Monday, delegates heard how the dumping of plastic waste into the world's oceans is causing at least $13 billion a year of damage, threatening marine life, tourism and fisheries.

"The key course of action is to prevent plastic debris from entering the environment in the first place, which translates into a single powerful objective: reduce, reuse, recycle," UNEP chief Achim Steiner said.

But others said that less talking and more action was needed, especially on the issue of wildlife crime.

- 'Highly organised crime' -

Wildlife protection agency CITES warned of the growing problem of poaching of Africa's animals, saying it was time for the same "frontline" tactics used against human traffickers and drug gangs.

"Unprecedented demand, wildlife crime and loss of habitat is destroying entire species and the building blocks of the ecosystem that we all rely upon," said John Scanlon, head of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

"Ultimately, this fight will be won or lost on the frontlines, whether in the field, the courtroom, or the market place -- not in a conference room," Scanlon said.

CITES warned Africa was suffering a "surge in poaching, in particular of elephants", and called for "even stronger law enforcement and demand-reduction efforts across multiple countries, to reverse the current dangerous trends."

Organised crime syndicates and rebel militia increasingly use poaching to fund insurgencies, reaping the benefits of multi-billion-dollar demand for ivory in China where it is used as decoration and in traditional medicines.

"We are fighting highly organised crime groups that target wildlife for profit," said Ben Janse Van Rensburg, head of enforcement for CITES.

"It remains vital for countries to recognise wildlife crime as a serious crime, to deploy the same tools and specialised techniques that we use to fight other organised crimes, such as human and drug trafficking."