Swedish climate activist Greta Thunberg opened a youth climate summit on Tuesday 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 summit in Glasgow, 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," 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.

The three-day event in Milan 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," said Thunberg.

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

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" 2 degrees Celsius.

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

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.

Host Britain says it wants the Glasgow summit to keep the 1.5C temperature goal in play, 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.