United Nations Secretary-General Ban Ki-moon warned international leaders on Monday that the Ukraine crisis could have profound implications for the integrity of a global treaty designed to prevent the spread of nuclear weapons.
Ukraine gave up its Soviet-era nuclear arsenal under the 1994 Budapest Memorandum, which it signed together with Britain, United States and Russia. It provided guarantees of Ukraine's sovereignty in exchange for a commitment, since fulfilled, to give up the country's nuclear weapons.
Russia, however, has seized and annexed Ukraine's Russian-majority region of Crimea following the fall in February of Ukraine's pro-Russian president after months of mass protests.
On Monday, Ukraine told its remaining troops to leave Crimea after Russian forces overran one of Kiev's last bases in the Black Sea region.
Ban's comments suggested concern that events in Ukraine could make some countries more reluctant to give up any nuclear weapons capabilities, or tempt others to pursue them.
Addressing a nuclear security summit in The Hague, Ban said security assurances were an essential condition for Ukraine's accession two decades ago to the nuclear Non-Proliferation Treaty (NPT), the 1970, 189-nation anti-nuclear weapons pact.
"However, the credibility of the assurances given to Ukraine in the Budapest Memorandum of 1994 has been seriously undermined by recent events," he told leaders from 53 countries, including US President Barack Obama and Russian Foreign Minister Sergei Lavrov.
"The implications are profound, both for regional security and the integrity of the nuclear non-proliferation regime," Ban said. But, "this should not serve as an excuse to pursue nuclear weapons, which will only increase insecurity and isolation."
Kenneth Brill, a former US envoy to the UN nuclear agency, said the United States in the past had "extended its nuclear umbrella in large part to keep those under it from seeking weapons of their own".
"These are security assurances of a kind and while not ones Ban Ki-moon would refer to directly are clearly, at least, in the background of his statement," Brill said in an email.
Nuclear bombs "a liability"
In The Hague on Monday, Obama sought support from European allies and China to isolate Russia over its seizure of Crimea.
Last week, Russia accused Western states of violating a pledge to respect Ukraine's sovereignty and political independence under the Budapest Memorandum, saying they had "indulged a coup d'etat" that ousted President Viktor Yanukovich.
But French Foreign Minister Laurent Fabius said it was Russia that violated the memorandum on security assurances, which also committed the signing countries to respect Ukraine's existing borders.
In his speech on nuclear security and other issues, Ban did not single out any side by name. He said commitments to undertake disarmament negotiations in good faith must be honoured and added: "So, too, must security assurances provided to non-nuclear-weapon states by nuclear-weapon states."
He called on states taking part in a NPT review conference next year to "address the legitimate interest of non-nuclear states in receiving unequivocal and legally-binding security assurances from nuclear-weapon states."
"We must ensure that nuclear weapons are seen by states as a liability, not an asset," Ban said.
Nuclear expert Miles Pomper at the Monterey Institute of International Studies said it was a long-standing goal of non-nuclear weapon states to win assurances in the NPT process that they will be protected from attack by nuclear-weapon states.
"Ban sees Ukraine as a serious problem in terms of providing security assurances to a non-nuclear weapon state," Pomper said.