During a joint news conference in the Netherlands, President Obama said that Russia does not top the list of security threats against the United States. (WhiteHouse.gov)

President Obama acknowledged Tuesday that Russian President Vladimir Putin’s annexation of Crimea would be difficult to reverse, but he dismissed Russia as a “regional power” that did not pose a leading security threat to the United States.

Concluding a summit here on nuclear security, Obama also warned that broader Russian military intervention in neighboring countries would trigger further economic sanctions that would disrupt the global economy but hit Russia the hardest.

“Russia is a regional power that is threatening some of its immediate neighbors — not out of strength but out of weakness,” Obama said in response to a reporter’s question about whether his 2012 election opponent, Mitt Romney, was right to characterize Russia as America’s biggest geopolitical foe.

“They don’t pose the number one national security threat to the United States,” Obama said in a news conference. “I continue to be much more concerned when it comes to our security with the prospect of a nuclear weapon going off in Manhattan.”

Defending his response to the Ukraine crisis, Obama dismissed criticism that a perception of weakness abroad had prompted Putin to seize Ukraine’s autonomous Crimea region this month, an act the United States and Europe have called a violation of both Ukrainian and international law. But Obama made clear that no military response is being contemplated, unless Putin pushes into NATO member nations on Russia’s western border.

“There’s no expectation that they will be dislodged by force,” Obama said in a news conference with Dutch Prime Minister Mark Rutte, who hosted the Nuclear Security Summit. “And so what we can bring to bear are the legal arguments, the diplomatic arguments, the political pressure, the economic sanctions that are already in place, to try to make sure that there’s a cost to that process.”

“I think it would be dishonest to suggest that there’s a simple solution to resolving what has already taken place in Crimea,” Obama added. “Although history has a funny way of moving in twists and turns and not just in a straight line.”

Obama has sought to galvanize European support here for broader sanctions against Russia should Putin expand his military campaign into eastern Ukraine or Moldova. Obama and leaders of six allied powers agreed to deepen Putin’s political isolation by effectively suspending Russia’s membership in the Group of Eight industrial nations.

Obama’s comments here Tuesday suggested that he does not believe that sanctions and diplomacy will be enough to persuade Putin to relinquish Crimea, the Black Sea peninsula that Russia has officially annexed in response to a hastily called referendum in which largely pro-Russian residents of Crimea voted to join the Russian Federation.

Obama also made clear that U.S. and European unity on matters such as future sanctions, certain to fall hardest on Europe’s economies, would likely be one of the few factors to influence Putin’s thinking.

“Although it could cause some disruptions to each of our economies or certain industries, what I’ve been encouraged by is the firmness and the willingness on the part of all countries to look at ways in which they can participate,” Obama said. “Our preference throughout will be to resolve this diplomatically. But I think we’re prepared, as we’ve already shown, to take the next step if the situation gets worse.”

Obama stressed that “we’re not recognizing what has happened in Crimea,” adding: “The notion that a referendum sloppily organized over the course of two weeks would somehow justify the breaking off of Crimea and the annexation by Russia . . . I think the overwhelming majority of the world rejects.”

Saying that “we are also concerned about further encroachment by Russia into Ukraine,” he warned of “ additional sanctions, additional costs, should Russia take this next step.” Those “sectoral” penalties would be “more broad-based sanctions that would impact entire sectors of the Russian economy,” such as energy, finance, arms sales or trade, he said.

“There is another path available to Russia,” Obama said. He urged Moscow to negotiate with Ukraine’s interim government, move Russian troops back from Ukraine’s borders and accept elections scheduled for May that will “allow the Ukrainian people to choose their leadership.”

He said it was “particularly important for all of us to dismiss this notion that somehow Russian speakers or Russian nationals inside of Ukraine are threatened and that somehow that would justify Russian action.” He dismissed as nonsense Putin’s repeated evocations of the breakaway region of Kosovo in the late 1990s, when thousands of Kosovars were slaughtered by Serbian forces. “There has been no evidence that Russian speakers have been in any way threatened” in Ukraine, Obama said.

William Branigin in Washington contributed to this report.