The United Nations' climate chief urged the coal industry on Monday to make dramatic changes to reduce its greenhouse gas emissions amid protests at a coal summit held on the sidelines of global climate talks.

As environmental activists clamoured outside the meeting venue, the UN's Christiana Figueres told the summit that the cheap but plentiful fossil fuel came today with a hefty and now intolerable price.

"While society has benefited from coal-fuelled development, we now know there is an unacceptably high cost to human and environmental health," she said.

"I am here to say that coal must change rapidly and dramatically for everyone's sake."

Figueres is in Warsaw for the annual round of negotiations under the UN Framework Convention on Climate Change (UNFCCC).

The talks, running until Friday, seek a way to a new, global deal by 2015 on curbing climate-altering greenhouse gas emissions.

The International Energy Agency (IEA) says coal accounted for 44 percent of carbon dioxide (CO2) emissions in 2011, the largest share, and remains the leading source of electricity and heat generation.

Activists and some delegates were angered by Poland's "endorsement" of the two-day coal summit.

The host body, the World Coal Association (WCA), went on the defensive on Monday.

"This is not an attempt to distract from the important work of these (climate) negotiations," WCA chief executive Milton Catelin told delegates.

The industry "accepts" that the burning of coal contributes to warming and that new technology is needed, he said, referring to CO2 capture and other initiatives.

There is "no solution to climate change that doesn't include some inclusion of coal," -- which is needed to power economic growth and poverty reduction, he told AFP.

"If coal is providing 40 percent of your electricity you cannot overnight switch to renewables. It's a decades-long process."

The summit brings together some of the world's biggest coal producers and consumers, policymakers, academics and NGOs to discuss the role of coal in the global economy and in the context of climate change, according to the WCA website.

'Coal summit' stokes protests

It is being held at the Ministry of Economy, just a few kilometres from the National Stadium hosting the climate talks.

Outside the ministry, Greenpeace activists hoisted huge anti-coal banners reading: "Who rules Poland? Coal industry or the people?"

Protesters on the roof waved the national flags of Canada, the United States, Japan, Britain, Germany, Brazil and the European Union (EU).

Others wore face masks, standing next to a pair of huge, plastic lungs to highlight the health consequences of coal pollution. China, the world's major coal user, has had several major smog alerts and health scares in its cities this year.

Police used a giant fire-engine crane to remove some protesters dangling from the building's facade from climbing cables.

Opening the meeting, Economy Minister and Deputy Prime Minister Janusz Piechocinski insisted that Poland had "with great consistency stuck to its international obligations to climate."

But, he said, "The largest coal deposits in the EU are in Poland, so over the next decade coal will remain an important fuel and can be a guarantor of energy for the entire EU."

According to the International Energy Agency, Poland was the world's ninth-biggest coal producer in 2012 and the 10th biggest producer of electricity from coal and peat.

A number of non-governmental organisations had urged Figueres to withdraw from addressing the coal summit.

But she stressed on Monday that her attendance was "neither a tacit approval of coal use, nor a call for the immediate disappearance of coal".

"The coal industry faces a business continuation risk that you can no longer afford to ignore," she said.

"By now it should be abundantly clear that further capital expenditures on coal can go ahead only if they are compatible with the 2.0 degree Celsius limit," she said, referring to the warming maximum sought by UN members.

The World Resources Institute (WRI) observer group said Monday that nearly 1,200 new coal-fired power plants have already been scheduled for development worldwide -- more than three-quarters of them in India and China.