BRUSSELS (REUTERS) - Climate activist Greta Thunberg accused European Union governments and institutions on Wednesday (March 4) of only "pretending" to urgently tackle the climate crisis.

EU leaders say they are spearheading some of the most ambitious climate change policies in the world, and on Wednesday the bloc's executive unveiled a planned climate law to reduce net greenhouse gas emissions to zero by 2050.

"When your house is on fire, you don't wait a few more years to start putting it out. And yet this is what the Commission is proposing today," Thunberg told the European Parliament's environment committee.

The 17-year-old Swedish activist said that, by fixing its climate law around an emissions target three decades in the future, the EU was ignoring reality.

"Here you are, trying to create laws and policies once again ignoring it. Pretending that your plan or policy disregarding the united science will somehow solve the biggest crisis humanity has ever faced," she said.

"Pretending that a law that no one has to follow... is a law. Pretending that you can be a climate leader and still go on building and subsidising new fossil fuel infrastructure," Thunberg said, drawing applause from European lawmakers.

The climate law would give the executive Commission powers to set tougher goals for governments every five years. But these powers would only kick in after 2030 - too late, according to environment groups.

"We will not be satisfied with anything less than a science-based pathway which gives us the best possible chance to safeguard the future living conditions for humanity and life on earth as we know it," Thunberg said.

The EU accounts for less than 10 per cent of the world's greenhouse gas emissions. But Thunberg said the bloc had a moral obligation to lead the way, because developed economies that have been polluting for longest should cut emissions first and fastest.

The European Parliament has banned external visitors for the next three weeks as a precaution against the fast-spreading coronavirus, but waived its own rule for Thunberg's visit.

Thunberg is in Brussels this week to meet EU leaders and lead a climate school strike on Friday.

"For over one-and-a-half years we have been sacrificing our education to protest against your inaction," Thunberg said.

She began missing school in 2018 to sit outside the Swedish parliament in a solitary protest. Since then, Thunberg has galvanised a global movement of young people, inspiring millions to take to the streets to demand that world leaders act urgently to tackle the climate crisis.