Teenage climate activist Greta Thunberg had made it half-way from Sweden to Chile by boat, train and electric car when next month's UN climate summit was unexpectedly scrapped.

But as a new venue was announced Friday for the gathering called COP25 -- this time in Spain -- the 16-year-old asked for a lift back across the Atlantic.

"As #COP25 has officially been moved from Santiago to Madrid I'll need some help," Thunberg tweeted.

"It turns out I've traveled half around the world, the wrong way:)"

"Now I need to find a way to cross the Atlantic in November... If anyone could help me find transport I would be so grateful," said the teenager, who refuses to fly because of the carbon emissions involved.

The call for help drew mixed reaction on social media.

Spain's Environment Minister Teresa Ribera said her country would help Thunberg make the transatlantic trip back, hailing her role in raising awareness of climate change.

But others mocked the young activist's uncompromising stance on air travel and encouraged her to go back to school.

Thunberg's highly publicized journey has so far involved crossing on a zero-emission sailboat from the coast of England to New York, travelling overland through North America by train and in an electric car borrowed from Arnold Schwarzenegger.

She was one of around 25,000 delegates expected in Santiago for the United Nations climate summit, until Chile pulled out as host this week due to deadly anti-government protests.

The Swedish teen activist Thunberg rose to prominence last year after she started spending her Fridays outside Sweden's parliament, holding a sign reading "School strike for climate."

Students across the world began emulating her campaign, leading to organized school walkouts and the rise of the "Fridays for Future" movement which targets government action on climate change.