Swedish teenage climate change activist Greta Thunberg has called for action, not praise, from US politicians as she joined other youth leaders in kicking off two days of meetings and speeches on Washington DC's Capitol Hill.

Key points: Greta Thunberg is in the US ahead of a global demonstration planned for Friday, when students and workers will appeal to leaders to enact climate saving policies

Greta Thunberg is in the US ahead of a global demonstration planned for Friday, when students and workers will appeal to leaders to enact climate saving policies Protesters want immediate action from politicians heading to the UN summit later this month

Protesters want immediate action from politicians heading to the UN summit later this month Ms Thunberg travelled to the US in a zero-carbon-emissions sailboat last month

She has met with former US president Barack Obama, who said he and Ms Thunberg were "a team".

This week's events are intended to raise awareness ahead of a global climate strike on Friday, when students and workers around the world are being encouraged to walk out of schools and jobs to demand action on climate change and pressure leaders due to attend the annual United Nations summit later this month.

"Please save your praise. We don't want it," Ms Thunberg told the Senate Climate Change Task Force.

"Don't invite us here to tell us how inspiring we are without doing anything about it, because it doesn't lead to anything."

She had a blunt message for Democratic senators at the invitation-only forum.

"I know you're trying … but just not hard enough. Sorry," she said.

Instead of listening to her and other teenagers, Congress should invite scientists and listen to their expertise on ways to slow a rise in global temperatures, Ms Thunberg said.

"This is not about us. This is not about youth activism," she said.

"We don't want to be heard. We want the science to be heard."

The 16-year-old has inspired millions of students and others to join her weekly strikes to demand action to fight climate change — dubbed "Fridays for Future", that inspired similar events in about 100 cities worldwide.

Last month she travelled to the US in a zero-carbon-emissions sailboat, to attend a global warming conference.

She has been nominated for a Nobel Prize.

Thunberg is calling on political leaders to enact policies to address climate change. ( Reuters: Sarah Silbiger )

Democratic senator Ed Markey, chairman of the task force, called the growing number of young protesters the new X-factor in politics and said their passion could play a significant role in next year's US presidential elections.

"2020 is going to be a referendum on climate change," Mr Markey told a news conference on Tuesday.

"It will be a referendum between Donald Trump and a whole new 'Green New Deal' direction."

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The Green New Deal is a non-binding congressional resolution Markey co-sponsored with Alexandria Ocasio-Cortez, a fellow Democrat, that aims to move the US economy away from fossil fuels within a decade.

Republican politicians and officials from the Trump administration have pilloried the plan as fantasy.

Mr Trump, one of the few world leaders who openly questions the science of climate change, has made a priority of rolling back Obama-era climate protections he says are not necessary and hurt the US economy.

Anaiah Thomas, a 17-year-old climate activist and member of US-based youth movement Zero Hour, told the senators they needed to take an urgent approach to climate change and support proposals like the Green New Deal.

"What's most important is to decide to take action today," she said.

"Not in five years. Not gradually. Not tomorrow."

AP/Reuters