The Bush administration called a deal to begin dismantling North Korea's nuclear weapons program a breakthrough, but the North's history of broken promises kept the celebrations to a minimum.
There was worry, too, that accommodation of North Korea would encourage brinksmanship by Iran or other would-be nuclear states.
The bargain among six nations gives North Korea energy, food and other aid in exchange for shuttering its main nuclear reactor.
It does not expressly require the North to give up existing weapons or testing now, and the agreement does not spell out how negotiators will resolve issues that have derailed previous pacts.
President George W Bush, who once labeled North Korea part of an "axis of evil," said the bargain is a promising first step toward getting rid of the North's nuclear weapons.
"These talks represent the best opportunity to use diplomacy to address North Korea's nuclear programs," Bush said in a cautious statement that stressed North Korea's obligations while saying little about what the United States would do.
Secretary of State Condoleezza Rice repeatedly urged patience.
"This is not the end of the story," Rice told reporters. Skepticism was wide and criticism swift, focusing on the troubled history of negotiations with the reclusive and unpredictable communist regime, and on what lessons Iran or any other countries with nuclear programs could draw.
On the right, a former top Bush aide joined conservative commentators in calling the deal naive.
Former UN Ambassador John Bolton said the agreement rewards North Korea for bad behavior while encouraging Iran to ignore international demands that it roll back its nuclear program and hold out for a better deal.
"I will be the saddest man in Washington" if Bush goes along with the agreement, Bolton told reporters.
"I think the agreement is fundamentally flawed."
At the conservative Heritage Foundation, analyst Bruce Klingner said the deal "reflects America's abandonment of several previously intractable negotiating positions".
North Korean leader Kim Jong-Il "used his characteristic mixture of military provocations, brinksmanship and crisis diplomacy to gain benefits for a return to the status quo ante and promises of future steps," Klingner said.
On the left, critics said Bush could have had the same deal years ago if he had not been so rigid in his approach to the North.
"This deal takes us back to the future," said Democratic Sen Joe Biden.
"The good news is that it freezes in place North Korea's nuclear program.
The bad news is that North Korea's program is much more dangerous to us now than it was in 2002, when President Bush rejected virtually the same deal he is now embracing."
Analysts say that at the start of the Bush administration North Korea probably had enough material to build one or two weapons, and now probably could produce at least 10.
North Korea stunned the world when it set off an underground nuclear test last October, proving its claim to possess weapons.
The North also test-fired a long-range missile last summer, showing that it has the theoretical ability to deliver nuclear weapons as far as the US West Coast.
"The deal announced today essentially freezes the North Korean weapons program, but it does nothing to actually force the country's leaders to give up any of the gains they've made in recent years," Democratic Sen John Kerry said.
"Unless the Bush administration continues to engage North Korea and stays focused on working diplomatically to find long-term solutions, we are likely to be dealing with this issue again in 12 months."
The nearly 15-year history of efforts to head off North Korean nuclear weapons is best described as one step forward, two steps back.
Both Republican and Democratic administrations have struck deals that the North sidestepped, ignored or boycotted.
After the North threw out international inspectors and threatened to quit the nuclear Nonproliferation Treaty, President Bill Clinton cut a deal in 1994.
The Bush administration has said the North cheated on that deal almost from the start.
Leery of previous offers and determined not to let the North use one-on-one talks with the US as a foil, the Bush administration banded with four other nations — Russia, China, Japan and South Korea — to launch new talks in 2003.
That process lagged while the North stepped up nuclear development, and seemed all but dead when the North announced in February 2005 that it had built a weapon.
In September of that year, however, the Bush administration was hailing an agreement on paper to renounce all weapons.
That is the agreement that Pyongyang boycotted for more than a year, before six-way talks resumed last fall.
Tuesday's deal begins to put some of the September 2005 deal in force, although the North's tests and continued development of nuclear material make the stakes much higher.