Washington and its allies face limited options in trying to halt North Korea's nuclear ambitions despite President Barack Obama's vow on Tuesday that Pyongyang will no longer be rewarded for provoking a crisis. Joined by South Korea's leader at the White House, Obama promised to end a cycle of letting impoverished North Korea create a nuclear crisis, then granting such concessions as food and fuel to get it to back down, only to see it renege later on its promises. Obama and South Korean President Lee Myung-bak achieved a level of unity that eluded Washington and Seoul during the era of their predecessors, George W. Bush and Roh Moo-hyun, when North Korea exploited gaps between a tough US stance and accommodating South Korean policies. “In the short run, that message is very powerful and sends a signal to the region that there's no daylight between the US and its allies,” said Nicholas Szechenyi of the Center for Strategic and International Studies in Washington. But over time, he said, “There's always a pendulum swing from pressure to dialogue and there aren't too many other options.” The United States and South Korea, as well as other regional players, are committed to enforcing UN Security Council actions approved last week that aim to crack down on North Korean weapons shipments and tighten curbs on its finances. North Korea, which last month conducted a nuclear explosion and missile tests in defiance of international pressure, said on the weekend it would start a uranium enrichment program and weaponize all its uranium in response to the new sanctions. Negotiations or war? Analysts reckon those threats from Pyongyang might be accompanied in coming weeks by new challenges, such as a North Korean attempt to pick a limited military fight along the two Koreas' land border or disputed maritime frontier. North Korea's defiant moves in recent weeks have made it easy for Obama to win support for confronting Pyongyang, with even normally reluctant China and Russia endorsing the UN sanctions. But as the crisis wears on, the question becomes: “How long can the Obama administration rally the international community in this tough approach and if North Korea fails to respond, what is the next step?” said Szechenyi. Bruce Klingner of the Heritage Foundation recommended Obama follow a “three-part menu” of punitive measures of various forms, attempts at negotiations, and defensive measures, including steps to expand and improve the US and allies' missile defense network. Obama held out to Pyongyang a path away from international isolation “through peaceful negotiations that achieve the full and verifiable denuclearization of the Korean peninsula.” A new study by the Center for a New American Security recommends bolstering US alliances in Asia, aggressive interdiction of North Korean shipping and close consultation with regional powers to prevent clashes from escalating. But Washington must also create “on-ramps” to negotiations by combining coercive steps with incentives to draw North Korea back to the negotiating table, it said. “Without further negotiations, the United States and its allies would be faced with the challenge of trying to destroy the DPRK's nuclear weapons capability through risky, costly, and uncertain means,” said the study.