The United States and China on Thursday agreed to keep up a tough sanctions regime on North Korea amid warnings that the reclusive nation is still managing to pursue its nuclear program, a US official said.

"Sanctions efforts in general were explicitly discussed" in talks between US Secretary of State John Kerry and his Chinese counterpart Wang Yi, the senior State Department official said.

"Notwithstanding any sweet sounding comments that foreign diplomats may think they've heard from the North Koreans, the troubling behavior by North Korea continues," the official told reporters at a briefing in New York.

And he stressed "North Korea's efforts to acquire a nuclear missile capability continue" despite a rigorous international sanctions regime.

"That is the problem that needs to be addressed through a combination of diplomatic and pressure means," he insisted.

Regional analysts and experts warned on Wednesday that North Korea's nuclear weapons program is developing beyond the international community's ability to rein it in with effective sanctions and export restrictions.

Even as Pyongyang's closest ally China announced an export ban to the North of technologies and goods with dual-use potential, experts questioned whether North Korea's weapons program hadn't already moved beyond its earlier dependence on external equipment and know-how.

"They are not at the start of this process anymore. They've been at it a long time," said Park Jiyoung, director of the Asan Institute's Science and Technology Policy Center.

"It's clearly likely that the North will try to go beyond its current nuclear capability ... (and) export controls can't stop that development," Park said.

North Korea has carried out three nuclear tests -- the last, and most powerful, in February this year, and six-party talks aimed at persuading Pyongyang to abandon its nuclear program have stalled.

"Both leaders agreed that it is important for us to coordinate closely to signal to North Korea that it has no alternative but to denuclearize," the US official said.

"And the Chinese decision to impose restrictions on what goes in and what comes out of North Korea, I think, is clearly indicative of their level of concern."

Satellite images suggest Pyongyang has restarted a plutonium reactor at its main Yongbyon nuclear complex and doubled its uranium enrichment capacity at the same site.

But the State Department official would not be drawn into either denying or confirming the reports.