Greta Thunberg took her campaign for greater action on climate change to the gates of the United Nations.

Swedish teenage activist Greta Thunberg took her weekly campaign for greater action on climate change to the gates of the United Nations on Friday, urging "everyone who cares about our future" to join her when world leaders gather in New York next month.

Thunberg, 16, started missing school on Fridays a year ago to protest outside the Swedish parliament, sparking a global climate strike movement known as Fridays for Future. She joined 14-year-old New Yorker Alexandria Villasenor on Friday, who began picketing outside the United Nations in New York December.

A couple of hundred of other young protesters supported them with signs that read "Help my home is on fire," "If you won't act like adults, we will" and "Science not silence." Their chants included "we are unstoppable, a better world is possible" and "sea levels are rising and so are we."

Wearing t-shirts that read "In Greta we trust," New York students Bianca Pilcher, 11, and Lila Sabag, 10, said they had one message for world leaders: act now.

"I don't want to live a short life, I want to live a long life and I want to have my world be healthy as well. I also want to experience having children, having grandchildren and them to not be like 'what have you brought me in to?'" said Pilcher.

Sabag said she wanted to help save the planet by trying "to not use much carbon dioxide or fossil fuels or like plastics and stuff." Then both girls chimed in together: "Always reuse."

Thunberg will speak at a Sept. 23 climate summit during the annual gathering of world leaders for the U.N. General Assembly. She sailed into New York Harbor on Wednesday in a zero-carbon emissions boat, completing a nearly 14-day journey from England.

In a statement when she arrived in New York, Thunberg said: "Everyone who cares about our future should join and strike on 20 and 27 September."

Thunberg met with U.N. General Assembly President Maria Fernanda Espinosa later on Friday.

"I think this summit needs to be some kind of breaking point, tipping point, where people start to realize what is actually going on," Thunberg told Espinosa.

"We have high expectations in you too, and all member states to deliver. And we are going to try to do our part to make sure that they have all eyes on them and they have put the pressure on them so they can't continue to ignore it."

U.N. Secretary-General Antonio Guterres has warned that the world faces a climate emergency and challenged leaders to come to the 193-member United Nations next month with concrete, realistic plans on how to better tackle the emergency.

"We absolutely need to keep the rise of temperature to 1.5 degrees Celsius to the end of the century and to be carbon neutral in 2050 and to have a 45 per cent reduction of emissions by 2030," he told reporters in France on Monday.

Thunberg intends to attend the 25th United Nations Climate Change conference in Santiago, Chile, in December, planning to make her way there without using air travel. She has taken a year off school to campaign for climate action in the Americas with plans to also visit Mexico and Canada.