Activist Greta Thunberg said the climate deal reached by world leaders in Glasgow was “very vague” and left open the prospect climbing global emissions and the expansion of fossil fuel infrastructure.

Countries at the Cop26 conference agreed to “phase down” coal use, rather than “phase out”, after an eleventh hour change led by China and India, and the 18-year-old climate campaigner said the deal struck “succeeded in watering down the blah, blah, blah”.

“Unfortunately it turned out just the way as I had expected, and that many others had also expected, they even succeeded in watering down the blah, blah, blah, which is quite an achievement,” the Swedish protestor told BBC Reporting Scotland on Monday.

“There is still no guarantee that we will reach the Paris Agreement. The text that it is now, as a document, you can interpret it in many, many different ways.

“We can still expand fossil fuel infrastructure, we can still increase the global emissions. It’s very, very vague.”

The Glasgow Pact, secured at the Cop26 talks, committed countries to take more climate action and featured the historic – if watered down – move against coal.

Ministers and negotiators at the UN summit agreed to get countries to strengthen their emissions-cutting targets for 2030 by the end of next year as part of the bid to limit dangerous warming climbing above 1.5C.

She welcomed the move to meet more frequently, but warned: “Yes, it’s good that they say that they’re going to increase their ambitions more often, about that doesn’t really mean much if they don’t actually increase their ambition, especially if they don’t fulfil that ambition, as they have proven so far now.”

Negotiators have also sent a signal on the shift away from the world’s dirtiest fuel, with the deal calling for efforts to accelerate the “phase down” of unabated coal, as well as the phasing out of inefficient fossil fuel subsidies.

It was the first time fossil fuels were mentioned in a Cop deal, and she told the broadcaster this was a “crucial step”.

The campaigner, who marched through the streets of Glasgow during the conference, added: “I think many people were surprised to learn we have had 26 Cops, and not once have we mentioned fossil fuels in the document up until now, then you start to wonder what have they been doing all this time.”

In the wake of the deal, Cop26 President Alok Sharma, who was close to tears on a couple of occasions during an hours-long final plenary, said the summit had met its key goal of keeping the 1.5C limit within reach.

Ms Thunberg admitted being the summit president was a “very difficult job” and she “would not trade my place for his”.

“He has a huge responsibility, not only that he has huge opportunity to make, not to make things right, I understand that this does not fall on just one individual, but he has an opportunity to push in the right direction. Being honest about where we are,” she said.