The United States and North Korea have been actively discussing the possibility of returning to denuclearization talks, raising the prospect of a new round of diplomacy even as Washington takes a tougher line against Pyongyang.

The countries’ nuclear envoys have been discussing the idea of “talks about talks,” according to multiple people with knowledge of the conversations. But they have not been able to agree on the logistics — in no small part because of North Korea’s continuing Ebola quarantine.

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“We want to test if they have an interest in resuming negotiations,” a senior U.S. administration official said, speaking on the condition of anonymity. “I think we’ve made it very clear that we would like to see them take some steps first.”

Those steps would include suspending work at North Korea’s nuclear facilities and pledging not to conduct any further nuclear tests, he said.

After years of broken North Korean promises, American negotiators are wary about taking Pyongyang at its word. But North Korea reacted angrily Sunday to the suggestion that it, not Washington, was the hurdle to resuming talks.

Relations between the United States and North Korea have explored new levels of acerbity in the weeks since Washington accused Pyongyang of being behind the devastating hack at Sony Pictures Entertainment, apparently in retaliation for “The Interview,” a movie that revolves around a scheme to assassinate North Korean leader Kim Jong Un.

In response, Washington has imposed new sanctions aimed at further limiting North Korea’s access to the international financial system, and Congress has been pushing for more. President Obama has been talking about the regime’s inevitable “collapse.”

When North Korea said it was willing to suspend nuclear tests if the United States and South Korea canceled annual military drills, the State Department turned down the offer, calling it “an implicit threat.” The immediate response surprised proponents of engagement, who say the offer, although unacceptable, represented an opening from North Korea that should have been considered.

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But behind the scenes, former and current officials have been discussing the idea of holding talks about how to resume the six-party negotiations aimed at persuading North Korea to abandon its nuclear ambitions.

Last month, a group of former American officials — including Stephen Bosworth and Joseph DeTrani, both of whom have a long history of dealing with North Korea — met in Singapore with Ri Yong Ho, North Korea’s vice foreign minister and lead nuclear negotiator.

The meeting was designed to check “the lay of the land,” according to one person familiar with the talks. Multiple Americans with knowledge of the various discussions spoke about them on the condition of anonymity because of the sensitivity of the issue.

The Singapore meeting resulted in the suggestion that Sung Kim, the U.S. special envoy for North Korea policy, meet with a North Korean counterpart. He was in Asia last week for meetings with Japanese, South Korean and Chinese officials, and he is understood to have raised the prospect of holding a meeting with the North Koreans in Beijing.

North Korea offered to send Ri to Beijing or suggested that Sung Kim meet in Pyongyang with Kim Kye Gwan and Kang Sok Ju, both more senior in the Foreign Ministry than Ri.

U.S. officials thought Kim’s and Kang’s ranks were better matched with Sung Kim’s position but did not like the “optics” of the American envoy traveling to Pyongyang, because it would have made the North Koreans look as though they were in the stronger position, according to the people close to the discussions.

Another big hurdle: North Korea still has strict quarantine rules in place following last year’s Ebola outbreak in West Africa. All people who have traveled outside the country — including, apparently, Ri, after his return from Singapore — are required to stay at home for 21 days.

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The few foreign arrivals have similarly been quarantined in Pyongyang; their only contact with the world outside their apartments is when a state-appointed doctor comes each day to check their temperatures, said foreigners living in the North Korean capital who have been subjected to the rules. This practice is continuing, even as the Ebola crisis subsides.

But the bigger problem is the crevasse between the countries’ starting positions.

“We have made it very clear publicly that we are open to engagement, substantive dialogue with North Korea about the issue of denuclearization,” Sung Kim said after his meetings in Beijing on Friday.

The United States’ fundamental position is still that “we’re willing to deal with the government that’s in power in North Korea if they will work with us sincerely towards credible negotiations on the nuclear issue,” he said.

North Korea responded angrily on Sunday.

In rejecting its invitation to host Sung Kim in Pyongyang, the United States was instead “working hard to shift the blame onto [North Korea], misleading public opinion by creating [the] impression that dialogue and contacts are not realized due to the latter’s insincere attitude,” an unnamed Foreign Ministry spokesman said in a statement carried by the official Korean Central News Agency.

David Straub, a former U.S. negotiator with North Korea, said the North Koreans “want to give the impression that it’s the Americans who are being unreasonable right now.”

But both sides have wanted to talk to each other for decades, he said. “The issue is not whether they want to talk but on what terms? What do they want to achieve?” Straub said. “The North Koreans have made it clear publicly and privately that they are a nuclear weapons state and that they intend to be a nuclear weapons state forever.”

Scott Snyder, a Korea expert at the Council on Foreign Relations, said getting back to talks is just the first test.

“The challenge of pursuing talks is how to resume them without accepting North Korea’s nuclear status, while simultaneously keeping up the pressure,” he said. “In the past, renewed dialogue has been accompanied by relaxation of pressure, especially from Beijing.”

The United States and North Korea have been negotiating directly and multilaterally over the latter’s nuclear program for more than two decades, with some successes — such as a 1994 normalization agreement and a 2005 six-party deal in which Pyongyang agreed to abandon its pursuit of nuclear weapons — but many more failures.

The six-party talks — including the United States, Russia, China, Japan and the two Koreas — began in 2003 and broke down in 2009 when North Korea said it would withdraw from the talks and resume its nuclear weapons program. A breakthrough seemed to come with a “Leap Day deal” in 2012, when the United States offered food aid, and North Korea said it would suspend uranium enrichment and allow in international inspectors. Less than six weeks later, North Korea launched a rocket, scuppering the deal, and then conducted its third nuclear test early in 2013.

Recent satellite imagery showed new activity at North Korea’s Yongbyon nuclear plant, possibly indicating that a reactor there was being restarted, according to analysts at the 38 North Web site. The reactor was the source of plutonium used in North Korea’s nuclear tests.