The Ukrainian leader made the comments at an EU summit on Thursday, in which he said "either Ukraine will have nuclear weapons, which will protect us, or we must have some kind of alliance".

He was recalling a conversation he had had with former US president Donald Trump about Ukraine giving up its nuclear arsenal after the collapse of the Soviet Union.

"This is a dangerous provocation," Putin said at a meeting with journalists from the BRICS group of emerging economies. "Any step in this direction will be met with a corresponding reaction.

"It is not difficult to create nuclear weapons in the modern world," he added.

"I do not know whether Ukraine is capable of doing it now, it is not so easy for Ukraine of today, but in general there is no great difficulty here."

Ukraine inherited the world's third-largest nuclear arsenal when the Soviet Union broke up in 1991. It surrendered it three years later after receiving security guarantees from Russia and the United States.

Those security guarantees, known as the Budapest Memorandum, required that the signatories respect Ukraine and the other ex-Soviet republics' territorial integrity and independence.

Zelensky said Russia had "violated this document", and that Ukraine was counting on joining NATO to avoid the need for nuclear weapons.

He made the remarks at a news conference following Thursday's summit.