Youth activists on Tuesday called out decades of "empty words and promises" from world leaders as they demanded action -- and money -- to tackle global warming ahead of a pivotal UN climate summit.

With just weeks to go before the COP26 meeting in Glasgow, seen as crucial to the viability of the Paris climate agreement, campaigners used the opening of a three-day event in Milan to lambast governments' failure to address the crisis.

Swedish wunderkind Greta Thunberg slammed governments for "shamelessly congratulating themselves" for insufficient pledges to cut emissions and promises of financing.

Hurling leaders' own words back at them, the 18-year-old laid bare to delegates at the Youth4Climate event the gap between words and action.

Swedish climate activist Greta Thunberg and Ugandan climate activist Vanessa Nakate called for urgent climate action, as well as funding for vulnerable countries to help them adapt
Swedish climate activist Greta Thunberg and Ugandan climate activist Vanessa Nakate called for urgent climate action, as well as funding for vulnerable countries to help them adapt AFP / Miguel MEDINA

"There is no Planet B, there is no planet blah, blah, blah," Thunberg said to warm applause.

Echoing a speech by COP26 summit host Boris Johnson in April, she continued: "This is not about some expensive politically correct dream of bunny hugging, or build back better, blah blah blah, green economy, blah blah blah, net zero by 2050, blah blah blah, climate neutral blah blah blah.

"This is all we hear from our so-called leaders: words, words that sound great but so far have led to no action, our hopes and dreams drowned in their empty words and promises," said Thunberg.

Some 400 youth activists from nearly 200 countries gathered in Milan will submit a joint declaration to a ministerial meeting at the end of the week, as a lead-in to COP26 in November in Glasgow.

Climate activist Greta Thunberg calls out decades of "empty words and promises" from world leaders as young people demand action -- and money -- to tackle global warming ahead of a pivotal UN climate summit
Climate activist Greta Thunberg calls out decades of "empty words and promises" from world leaders as young people demand action -- and money -- to tackle global warming ahead of a pivotal UN climate summit Ecological transition ministry

"Our leaders' intentional lack of action is a betrayal of all present and future generations," said Thunberg.

She said governments had been "shamelessly congratulating themselves while still failing to come up with the long overdue funding" for developing nations.

Ugandan youth activist Vanessa Nakate echoed Thunberg's exasperation at leaders' lack of urgency.

"How long must children sleep hungry because their farms have been washed away, because their crops have been dried up because of the extreme weather conditions?" she asked attendees.

COP26 President Alok Sharma told delegates on Tuesday that the summit was a chance for nations to make good on their finance pledges
COP26 President Alok Sharma told delegates on Tuesday that the summit was a chance for nations to make good on their finance pledges AFP / MIGUEL MEDINA

"How long are we to watch them die of thirst and gasp for air in the floods? World leaders watch this happen and allow this to continue."

COP26 is vital for the effective implementation of the 2015 Paris Agreement, which saw countries commit to limit global temperature rises to "well below" 2 degrees Celsius.

But six years after the accord was struck, countries still haven't agreed how it will work in practice.

Among long-overdue issues still outstanding for COP26 is how each country's carbon cuts will be counted, as well as how the fight against climate change is financed.

Nations already suffering from extreme floods, droughts and storms supercharged by rising seas have called on developed countries at COP26 to make good on a decade-old promise to provide $100 billion each year to help them recover and adapt.

COP26 President Alok Sharma told delegates on Tuesday that the summit was a chance for "developed countries to make good on their promise of money to support developing countries dealing with rising impact of climate change".

Host Britain says it wants the Glasgow summit to keep the 1.5C temperature goal viable, specifically by seeking a global agreement to phase out coal power.

However the United Nations this month said that the latest round of country emissions reductions plans still puts Earth on course for a "catastrophic" 2.7C of warming.

"It's time for our leaders to stop talking and start acting, it's time for the polluters to pay, it's time to keep promises," said Nakate.

"No more empty promises, no more empty summits, no more empty conferences. It's time to show us the money."