NEW YORK -- Russia's brazen takeover of the Crimean Peninsula and the inherent disregard for an international agreement Moscow signed, vowing to respect Ukraine's borders in exchange for the smaller nation's abandonment of nuclear weapons, have Western world leaders worried that the superpower could just as easily disregard the nuclear nonproliferation pacts that marked the now-distant U.S.-Russia diplomatic "reset."

"We are very concerned about the impact on nonproliferation more generally if Russia is prepared to tear up the commitments it makes," a senior Western diplomat told CBS News.

President Obama said at the Nuclear Security Summit this week in the Netherlands that he was now "much more concerned, when it comes to our security, with the prospect of a nuclear weapon going off in Manhattan."

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It was an effort to refocus the world's attention on what the White House calls one of the "greatest threats" to security.

Mr. Obama and U.N. Secretary-General Ban Ki-moon had appealed repeatedly to Russian President Vladimir Putin to refrain from annexing Crimea - with little success.

The U.S. and the European Union have imposed sanctions on Russian banks and individuals, while the U.N. General Assembly voted Thursday to support Ukraine's new government and call the referendum that resulted in the annexation illegal. The Security Council is now working to get more observers into Ukraine and Crimea to guard against attacks on civilians.

While the efficacy of those and future sanctions - and their potential ramifications for both Russia and the West - have been the focus of the public discussion, the fallout of the Crimea crisis on global nuclear nonproliferation has remained largely within diplomatic corridors. But officials within those corridors are getting increasingly worried about the impact.

"Commitments to undertake disarmament negotiations in good faith must be honored," Ban told the Nuclear Security Summit at The Hague. "So, too, must security assurances provided to non-nuclear-weapon states by nuclear-weapon states."

"The credibility of the assurances given to Ukraine in the Budapest Memorandum of 1994 has been seriously undermined by recent events," Ban added, referring to the agreement Moscow signed with Kiev, which saw Ukraine give up its nuclear weapons program in exchange for a promise that its borders would be respected.

The implications, the secretary-general went on to say, are profound: "This should not serve as an excuse to pursue nuclear weapons, which will only increase insecurity and isolation."

The actions in Ukraine, combined with the general contentiousness between the White House and the Kremlin, make it more likely to have a negative impact on the global "architecture" of nuclear security, diplomats at the U.N. fear.

The crisis may even extend to the Iran nuclear negotiations, which U.N. officials and diplomats say are going increasingly sour. They are a part of Mr. Obama's fence-mending agenda with Saudi Arabia, where the president traveled Friday.

Last week, Russian Deputy Foreign Minister Sergey Ryabkov said that Russia hoped to not abandon Iranian nuclear talks to "raise the stakes" but may have to do so in response to the latest round of sanctions.

During Thursday's vote, Ukraine's representative at the General Assembly, Andrii Deshchytsia, pointed to the consequences of the crisis in Ukraine on nuclear security.

"Six months ago, we were preparing to have a trilateral meeting in The Hague between the leaders of Ukraine, USA and Russia to celebrate the 20th anniversary of Ukraine's nuclear free success story," said Deshchytsia. "Yet this plan has been crushed almost overnight by the Russian aggression against Ukraine and its annexation of Crimea."

And Security Council diplomats agree. U.K. Ambassador to the U.N. Mark Lyall Grant underscored the bigger issue.

"They have broken international agreements," he said. "And that is serious because it is a little step further toward lawlessness and a breakdown of the international order. That is what is a deep concern to the countries neighboring Russia but should be a deep concern to every member of the U.N."

But there is time for Mr. Obama to salvage the nuclear security issue, Western diplomats say. At the nuclear summit, the president announced that world leaders will convene at a nuclear summit in the U.S. in 2016.

More significantly, Mr. Obama will have another shot at making Russia a partner on nonproliferation of nuclear weapons later this year.

When world leaders gather this September during the General Assembly debate, the Security Council's president will be the U.S., a fact that results from U.N. rules and alphabetical order of the presidency, which sets the agenda.

Like in 2009, when Mr. Obama was president of the Security Council and garnered unanimous support (including from Russia) for a resolution that called on nations to disrupt illicit trafficking in nuclear materials, Mr. Obama may have another shot at making Russia a partner in nonproliferation