BEIJING - Having coaxed North Korea to shut an aged reactor, disarmament talks resuming this week face the harder task of persuading Pyongyang to loosen its grip on broader atomic ambitions it has long held vital to survival.

North Korea locked its Soviet-era Yongbyon nuclear plant and allowed U.N. atomic monitors back to the site in July, following a Feb. 13 deal made at six-party talks in Beijing.

In return, Pyongyang has received shiploads of heavy fuel oil and held bilateral talks with the United States that could eventually bring the impoverished fortress state out of diplomatic isolation.

But having reached that milestone, negotiators meeting from Thursday must begin to line up a daunting set of decisions -- especially how to disable Yongbyon and what details North Korea must disclose in its declaration of atomic activities.

"If we fail to come to an agreement we will go back to where we started," chief North Korean negotiator Kim Kye-gwan said on arriving in Beijing on Tuesday.

A South Korean diplomat, speaking anonymously, said the four days of talks were unlikely to yield final agreement on next steps. Any one of the unresolved issues could bog down or derail talks, said several experts, especially with claims swirling around Washington that North Korea gave nuclear help to Syria.

"The closure of Yongbyon wasn't the key to this dispute. It was the prelude to resolving the key issues," said Zhang Liangui of the Central Party School in Beijing.

"The key will be whether North Korea will agree to revealing its nuclear weapons in the declaration and how it will explain its uranium enrichment activities."

Late last year, two months after North Korea's first nuclear test blast heightened international pressure on Pyongyang, prospects for progress in the standoff were clouded.

SPUTTERED TO LIFE

But the talks between North and South Korea, China, the United States, Japan and Russia sputtered back to life following breakthrough two-way meetings between Washington and Pyongyang.

Envoys agreed to give North Korea 50,000 tonnes of heavy fuel oil or equivalent aid in return for shutting Yongbyon, which can make the plutonium that the North used in its test blast.

If Pyongyang completes the next phase of disarmament tasks, it will get another 950,000 tonnes of fuel oil or equal aid.

But the February deal left big uncertainties that North Korea could exploit to avoid hard choices, said former U.S. diplomat Joel Wit, who helped forge an earlier disarmament deal with Pyongyang.

"If you look at any one of these agreements, there are holes you could drive a truck through," he said of the recent deals.

While North Korea has agreed to fully declare all nuclear activities, negotiators have to decide how much detail it must share about its plutonium stockpile and any work in uranium enrichment.

The negotiations are also clouded by speculation that a Sept. 6 Israeli air strike on Syria may have been triggered by concerns that Syria had received nuclear help from North Korea -- a claim that Pyongyang has denied.

On Monday, U.S. Secretary of State Condoleezza Rice hinted that North Korea could be dropped from a U.S. terrorism blacklist before fully accounting for the Japanese citizens it abducted in the 1970s and 1980s, a move that could upset Japan.

U.S. envoy Chris Hill said in Tokyo on Tuesday that "no decisions have been made" on the blacklist.