The US will take action on greenhouse gases and engage with other countries on the climate emergency despite Donald Trump’s rejection of international cooperation, a delegation from the US Congress has told the UN climate conference in Madrid.

Nancy Pelosi, the speaker of the House, struck a defiant stance on Monday, declaring: “Congress’s commitment to action on the climate crisis is iron-clad. This is a matter of public health, of clean air, of clean water, of our children, of the survival of our economies, of the prosperity of the world, of national security, justice and equality. We now must deliver deeper cuts in emissions.”

Her rallying call came as developing countries accused the US president of “ecocide” and the UN secretary-general said the world’s biggest emitters were falling behind.

Ministers and officials from more than 190 countries gathered in Madrid on Monday for the start of two weeks of talks aimed at ironing out technical details of the 2015 Paris agreement, which needs to be completed for nations to focus on the progress on cutting carbon emissions.

“We are outraged by the dithering and retreat of one of the most culpable polluters from the Paris agreement,” Lois Young, Belize’s permanent representative to the UN and chair of the Alliance of Small Island Developing States, told the conference.

“In the midst of a climate emergency, retreat and inaction are tantamount to sanctioning ecocide. They reflect profound failure to honour collective global commitment to protect the most vulnerable.”

António Guterres, the UN secretary general, also called for more action from the US, the world’s second biggest emitter after China, warning that some of the world’s biggest economies were failing to cut carbon fast enough.

“We see already strong commitment from many governments and the business and financial community – the problem is that the most important polluters, the countries that have the biggest [emissions of] greenhouse gases are lagging behind.”

He praised the EU for its plans to cut emissions drastically by 2030, and cited China, India, Japan and the US as countries needing to join in to meet the challenge.

“This is also an issue for public opinion, for youth and civil society, for cities and regions – we see everywhere a new determination that makes me hopeful,” he added. “I’m hopeful, but not yet entirely sure as there is still a long way to go and we are still lagging behind.”

The opening of the two weeks of talks, called COP25, was a more subdued affair than usual because the venue was forced to change a few weeks ago from Santiago in Chile after violence and political upheaval erupted.

Activists and campaigners assembled inside the conference and outside, but there was a lack of the pageantry and passion that have characterised recent COPs, galvanised by the signing of the Paris accord in 2016, seen as paving the way to international cooperation and preventing dangerous temperature rises.

Progress since then has been slow, and emissions have continued to rise while most countries have failed to propose the tougher targets needed to fulfil the goals of the Paris agreement. Meanwhile, scientific warnings have grown more urgent and extreme weather events have struck home the dangers of climate chaos.

Several countries have also elected leaders hostile to the Paris accord and action on emissions, including Australia, Brazil and the US.

Trump deposited the legal instruments needed to formally withdraw from the Paris agreement last month, which will take effect next November. But the US will continue to have a seat at the table of UN climate negotiations, as he has not withdrawn from the foundational treaty, called the UN Framework Convention on Climate Change.

Pelosi and other members of the US congressional delegation – which has no formal role at the talks – also played down perceptions of a rift between more leftwing Democrats who favour a “green new deal” that would eliminate emissions from 2030, and those behind a bill that would legislate for net zero carbon by 2050.

Frank Pallone, sponsor of the 2050 bill, said there was “nothing inconsistent” about the two, as moving to renewable energy by 2030 would allow other sources of emissions, such as agriculture and some industries, to take longer to reduce. “None of this is inconsistent – we need to dispel the notion that we are not incorporating the green new deal,” he said.