"The science is clear. It is irrefutable and it is alarming," Kerry told a climate conference in Majuro in the Marshall Islands in a video address from Washington.
"If we continue down our current path, the impacts of climate change will only get worse."
Kerry said without strong, immediate action, the world would experience threats to critical infrastructure, regional stability, public health, economic vitality, and the long-term viability of some states.
Washington's top diplomat was addressing climate experts meeting on the eve of the Pacific Islands Forum (PIF) in the Marshall Islands, a low-lying nation where rising seas threaten to swamp many atolls.
"I stand with you in the fight against climate change," he pledged, adding the issue was a global crisis that was beyond one country to fix and needed urgent global action.
"If we act together, there is still time to prevent some of the worst impacts of climate change," he said. "But the people of the Pacific Islands know as well as anyone that we also need to prepare communities for the impacts that are already being felt."
Kerry is not attending the PIF, with Secretary of the Interior Sally Jewell representing the United States instead.
Hedegaard expressed concern that some countries may try to delay a 2015 deadline for implementing reductions in emissions and increasing reliance on alternative energy sources.
She said Europe and the Pacific island nations would work together to push the international community to honour the deadline.
"We have to make a joint pressure to say the world is already more than late (in addressing climate change)," she told the conference in the capital Majuro.
"2015 must be taken seriously."
The 15 PIF nations include islands states such as Kiribati, Tuvalu and the Marshalls, where many atolls are barely a metre (three feet) above sea level and risk being engulfed by rising waters.
The plan is to then present the declaration to UN Secretary General Ban Ki-moon at the General Assembly meeting in New York at the end of September, "to reenergise the international community".