Seoul, South Korea ? South Korea honed a compromise proposal Saturday to resolve the standoff over North Korea’s nuclear program, but Pyongyang warned the situation remained “serious and unpredictable.”

The pace of diplomacy was picking up, with a South Korean diplomat in Russia — an ally of the north, and talks slated Monday and Tuesday in Washington among the United States, the South and Japan. The South Korean proposal was expected to dominate a joint strategy session.

In advance of that regular session to review policy toward the North, the Bush administration had not budged from its demand that the the communist regime in the North unilaterally abandon its nuclear ambitions before Washington considers a next step.

Details of the Seoul plan were scant, but media reports suggested the proposal would require concessions from both Washington and Pyongyang.

South Korea’s deputy foreign minister arrived in the Russian capital, saying Russian assistance was essential.

“Our government thinks that the role of the Russian government in the process of peaceful resolution of this problem is very important and constructive,” Kim Hang-kyung said.

Russian President Vladimir Putin has moved to reinvigorate Moscow’s strong Soviet-era ties with North Korea. Moscow has said it wanted to link its trans-Siberian railroad with a railway being rebuilt between North and South Korea.

Seoul’s diplomatic offensive underlines its new drive to mediate between the United States, its key ally, and neighboring North Korea, its erstwhile enemy. But brokering a deal won’t be easy.

The United States refuses to talk until the North scraps its nuclear programs. And North Korea insists Washington must take the first step by signing a nonaggression pact promising not to attack the country.

“There is no reason why the U.S. should not accept the proposal, the best way for peaceful solution,” the North’s state-run news agency KCNA said.