UN Secretary-General Antonio Guterres and Brazilian President Luiz Inácio Lula da Silva arrived at the talks as they get to crunch time.
Lula's tentative schedule included meetings with negotiators for the European Union, emerging nations in Latin America, the Middle East and Asia, and hard-hit small island nations and African countries.
"The arrival of President Lula and Antonio Guterres is hugely significant and suggests the hosts think we should have some kind of deal passed today," said Mohamed Adow of the Kenyan environmental organisation Power Shift Africa.
"You don't generally have the president coming into the talks midway through the second week, without something to show for it."
Raising the possibility of a historic outcome, Greenpeace Brazil executive director Carolina Pasquali said: "The COP is nearing the endgame and the joint arrival of both Lula and Guterres gives a clear political signal that they mean business."
The two-week UN summit in the Amazon city of Belem has brought nearly 200 countries together to try to ratchet up multilateral action to limit climate change, despite the absence of the US, the top historic greenhouse gas emitter.
But rifts on key issues remain, posing a fresh test of the international will to slow global warming.
Host Brazil, hoping to buck the trend in which recent climate summits ran well past deadline, seeks to endorse a package of agreements later on Wednesday, and the outstanding issues on Friday.
The first version of the deal published on Tuesday had presented a range of options that split opinion. Brazil and around 80 other supportive nations want to agree something that helps spur action on a 2023 agreement made at COP28 to transition away from fossil fuels, the main source of greenhouse gas emissions.
However, the idea of creating a roadmap to help guide that transition had so far been rejected by others, Brazil's COP30 President Andre Correa do Lago said.
Pacific island nation Vanuatu's climate minister Ralph Regenvanu told Reuters Saudi Arabia was one of those opposed to the fossil fuel plan.
Saudi Arabia did not immediately respond to requests for comment.
"I think it's going to be very difficult ... because we've got blockers," Regenvanu said.
Other island nations said the issue was vital.
"We're going to have to fight tooth and nail. There are many parties who have already said that they do not want that in the text at all," Marshall Islands climate envoy Tina Stege told Reuters.
A coalition of 100 organisations, including companies like Volvo and Unilever, sent a letter to the COP30 presidency expressing support for a roadmap to transition away from fossil fuel use, saying it would help countries and businesses plan the shift to cleaner energy.
Other contentious issues in the package include pinning down how rich countries will provide finance to poorer countries to switch to clean energy, and what must be done about a gap between promised emissions cuts and those needed to stop temperatures rising.
Poorer countries already bearing the impacts of global warming are rallying for a strong outcome.
"We want ambition on finance. We want ambition on adaptation. We want to see ambition on the transition," Jiwoh Abdulai, Sierra Leone's climate minister, told Reuters.
"And we want to ensure that we live here on a path that is sustainable, not just for this generation, but for future generations."
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