Swedish activist Greta Thunberg opened a youth climate summit by lambasting three decades of government inaction, accusing world leaders of having "drowned" future generations with "empty words and promises".

Speaking weeks ahead of a crunch UN climate meeting in Glasgow, Ms Thunberg accused governments of "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 in Milan the gap between words and action.

"There is no Planet B, there is no planet blah, blah, blah," Ms 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 Ms Thunberg.

The three-day event in Italy gathers some 400 youth activists from nearly 200 countries, who 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.

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

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 Ms 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.

"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."

Vanessa Nakate cries after her speech as Greta Thunberg comforts her at the summit

COP26 is being billed as vital for the continued viability of the 2015 Paris Agreement, which saw countries commit to limit global temperature rises to "well below" 2C.

The landmark deal aims for a safer warming cap of 1.5C.

But six years after the accord was struck, countries still have not 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 that "now is the time for the developed countries to make good on their promise of money to support developing countries dealing with rising impact of climate change".

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