The UN chief said Monday that the failure to live up to promises to Ukraine in return for giving up its nuclear weapons should not discourage others from abandoning their atomic arsenals.

Opening a major summit of world leaders devoted to nuclear security, Ban Ki-moon said Russia's absorption of Crimea had "seriously undermined" the 1994 Budapest Memorandum, under which Ukraine handed over its Soviet-era nuclear weapons in return for guarantees of sovereignty from Western powers and Moscow.

"In the case of Ukraine, security assurances were an essential condition for its accession to the Nuclear Non-Proliferation Treaty," Ban told the assembled group of 53 world leaders, including US President Barack Obama.

"However, the credibility of the assurances given to Ukraine in the Budapest Memorandum of 1994 has been seriously undermined by recent events," the UN leader added.

Saying the implications were "profound" both for regional security and for the nuclear non-proliferation regime, Ban said: "This should not serve as an excuse to pursue nuclear weapons, which will only increase insecurity and isolation."

The summit, the third bi-annual Nuclear Security Summit (NSS), was called to advance efforts towards increased nuclear security and reducing the world's non-military nuclear materials but has been overshadowed by the Ukraine crisis.

Obama has called an emergency meeting of the Group of Seven (G7) top industrial powers on the sidelines of the summit to debate further measures against Russia, which could find itself expelled from the wider G8 group.

Ban's comments followed similar warnings from Canada's Prime Minister Stephen Harper, who said Saturday that undermining the Budapest Memorandum could cause aggrieved nations to "arm themselves to the teeth".

For his part, Ukraine's interim prime minister Arseniy Yatsenyuk has complained: "In 1994, my country abandoned, by its free will, the nuclear weapon ... and what has happened (since is that) one of the signatories of this... Budapest Memorandum invaded my country."

Leaders at the NSS were expected to hammer out a so-called "nuclear security architecture" plan to ensure that nuclear material does not fall into the wrong hands.

The summit is the brainchild of Obama who in 2009 called nuclear security "one of the greatest threats" to international peace.

Dutch Prime Minister Mark Rutte, who is hosting the talks, said the challenge of preventing nuclear terrorism was growing "more urgent, not less".

"We cannot rest on our laurels.... We have to aim higher. Even as we speak, there are still almost 2,000 tonnes of weapons-usable material in circulation worldwide, and its security has to be our constant concern," said Rutte, in opening remarks broadcast to reporters.