Rafael Grossi, head of the International Atomic Energy Agency, spoke to Reuters a day before he is expected to travel to Europe's largest nuclear power station in the Russian-occupied Zaporizhzhia region of southeastern Ukraine.
Grossi has been pushing for a safety zone to be created at the plant to prevent a possible nuclear disaster as Russia and Ukraine have accused each other of shelling the site of the power station.
"We are making some adjustments on the proposals that we are putting on the table," Grossi said in an interview in the central Ukrainian city of Dnipro.
"I am confident that it might be possible to establish some form of protection, perhaps not emphasising so much the idea of a zone but on the protection itself: what people should do, or shouldn't do to protect (the plant) instead of having a territorial concept."
The contours of the proposed deal have not been made public.
Diplomats say Grossi's latest proposal no longer includes a defined radius around the plant to mark the zone.
Ukraine does not want a deal that will in effect recognise or allow a Russian military presence at the plant.
Other elements of Grossi's plan include no firing at or from the plant and the removal of heavy weapons.
"I am not giving up in any way. I think on the contrary we need to multiply our efforts, we need to continue," Grossi said.
He said there had been increasing military activity in the region without giving details.
Grossi, who met President Volodymyr Zelenskiy on Monday, described the situation at the plant as "very dangerous" and very unstable.
It has lost its external power supply six times since Russia's invasion, forcing emergency diesel generators to kick in to cool its reactors.
Grossi said the water level in a nearby reservoir controlled by Russian forces was another potential danger.
Water supplied by the reservoir is used to cool the reactors.
"If the reservoir level goes down beyond a certain level, then you don't have water to cool down the reactors, and we have seen especially in January that the levels of the water were going down significantly. They recovered somehow in the past few weeks," he said.
The IAEA has had its own monitors stationed at the Zaporizhzhia plant since last year.
Grossi blamed a recent delay in their rotation on a row between Russia and Ukraine over the route they were supposed to take.
"We had an agreed route. All of a sudden that route was not agreed anymore... It took an awful lot of time to come to an agreement," he said.
