Pacific island nations are in “grave danger” from rising sea levels and the world must “answer the SOS before it is too late”, the UN chief has warned during a visit to Tonga.

The UN secretary general, António Guterres, urged the world to “look to the Pacific and listen to the science” as he released two new reports on the sidelines of the Pacific Islands Forum, the region’s most important annual political gathering.

Sea-surface temperatures in the south-west Pacific have risen three times faster than the global average since 1980, according to a regional report compiled by the World Meteorological Organization (WMO) and released on Tuesday.

It also found that marine heatwaves in the region had roughly doubled in frequency since 1980 and become more intense and longer-lasting.

The report said 34 mostly storm or flood-related “hydrometeorological hazard events” in the south-west Pacific last year led to more than 200 deaths and affected more than 25 million people.

In a second report published on Tuesday, the UN’s climate action team warned that the climate crisis and sea-level rise were “no longer distant threats”, especially for the Pacific’s small island developing states.

The Intergovernmental Panel on Climate Change (IPCC) concluded with high confidence in 2021 that the global mean sea level was rising at rates unprecedented in at least the last 3,000 years as a result of human-induced global warming.

But the new UN report, titled Surging Seas in a Warming World, said: “Since then, emerging research on climate ‘tipping points’ and ice sheet dynamics is raising alarm among scientists that future sea-level rise could be much larger and occur sooner than previously thought.”

Sea-level rise is caused by melting land ice and the expansion of seawater as it warms. Guterres told reporters in Tonga on Tuesday that it had “unparalleled power to cause havoc to coastal cities and ravage coastal economies”. He said Pacific islands were “uniquely exposed” because 90% of people lived within 5km of the coast and about 50% of infrastructure was located within 500 metres of the sea.

If the world heated to 3C above preindustrial levels, which is roughly what is expected under current policies, Pacific islands “can expect at least 15cm of additional sea-level rise by mid-century, and more than 30 days per year of coastal flooding in some places”, Guterres said.

“I am in Tonga to issue a global SOS – Save Our Seas – on rising sea levels,” he said.

“This is a crazy situation. Rising seas are a crisis entirely of humanity’s making, a crisis that will soon swell to an almost unimaginable scale, with no lifeboat to take us back to safety. But if we save the Pacific, we also save ourselves.”

Seeking to build momentum for action before the Cop29 climate summit in Azerbaijan in November, Guterres called on global leaders to drastically slash global emissions and pursue a “fast and fair” phase-out of fossil fuels.

He called for a “massive” increase in finance and support for vulnerable countries, arguing: “We need a surge in funds to deal with surging seas.”

The issue is expected to be one of the key items on the agenda at Cop29 due to the imminent expiry of the previous commitment by wealthy countries to mobilise US$100bn of climate finance a year.

On Tuesday, an alliance of civil-society groups called on Australia to “step up and support our Pacific neighbours on the frontlines of the climate crisis”.

The groups, which include ActionAid Australia and Oxfam Australia, urged Australia to declare its support for a new US$1tn global climate finance goal, arguing this could prompt other wealthy nations to step up.

“Australia and New Zealand’s climate finance contributions are falling short of need,” the organisations said in a new report, Seizing the Moment: A New Climate Finance Goal that Delivers for the Pacific.

The report said Australia had committed to provide $3bn in the five-year period to 2025, but this was “well short of its estimated fair share of the US$100bn goal, which is A$4bn per year”.

Rufino Varea, the regional director of the Pacific Islands Climate Action Network, said Pacific communities were “enduring some of the world’s worst climate impacts despite contributing the least to the crisis”.

The executive director of ActionAid Australia, Michelle Higelin, said: “We can’t tinker around the edges when it comes to climate finance. The climate crisis is already pushing Pacific countries into excruciating debt and deepening gender inequality.”

The Australian prime minister, Anthony Albanese, is due to arrive in Tonga on Tuesday evening for the annual Pacific Islands Forum leaders’ meeting. The regional grouping brings together Australia, New Zealand and 16 other Pacific nations.