MOSCOW - Russia and China urged North Korea yesterday to return to the negotiating table on the fate of its rogue nuclear programs - an unusual joint appeal from two Security Council members who have resisted more punitive U.S. measures against Pyongyang.

The appeal, which also expressed "serious concern" about tensions on the Korean peninsula, came just hours after North Korea warned of a "thousand-fold" military retaliation against the United States and its allies if provoked. The United States, meanwhile, called on Pyongyang to stop its saber-rattling.

That Chinese and Russian leaders used their meetings in Moscow to jointly pressure North Korea appeared to be a signal that Moscow and Beijing are growing impatient with Pyongyang's stubbornness.

Northeastern China and Russia's Far East border North Korea, and Pyongyang's unpredictable actions have raised concern in both countries.

With both Washington and Pyongyang exchanging near-daily rhetorical salvos, Russia and China appeared to be positioning themselves as moderators in the dispute.

After meetings at the Kremlin, Chinese President Hu Jintao joined Russian President Dmitry A. Medvedev in urging a peaceful resolution of the Korean standoff and the "swiftest renewal" of the now-frozen talks involving their countries as well as North and South Korea, Japan, and the United States.

"Russia and China are ready to foster the lowering of tension in Northeast Asia and call for the continuation of efforts by all sides to resolve disagreements through peaceful means, through dialogue and consultations," the statement said.

The comments - in a lengthy statement that discussed a host of global issues - included no new initiatives, but appeared to be carefully worded to avoid provoking Pyongyang. In remarks after their meetings, Medvedev made only a brief reference to North Korea and Hu did not mention it.

Hours earlier, North Korea reacted angrily to President Obama's declaration that North Korea was a "grave threat" to the world. Obama spoke during a summit with South Korean President Lee Myung Bak in Washington.

"If the U.S. and its followers infringe upon our republic's sovereignty even a bit, our military and people will launch a one hundred- or one thousand-fold retaliation with merciless military strike," the government-run Minju Joson newspaper said in a commentary.

Yesterday, Lee told an audience at George Washington University, where he received an honorary degree, that South Korea would not allow nuclear weapons in North Korea under any circumstances.

But he also held out the possibility that a nuclear-free North Korea could gain peace and prosperity.

"North Korea must understand that it is in their best interests to fully give up their nuclear-weapons ambitions," Lee said. "When North Korea takes meaningful steps toward peace and dialogue, Korea, as well as the rest of the international community, stands ready to extend a helping hand."

Both China and Russia long resisted efforts by Washington to impose stricter sanctions or other punitive measures on North Korea. But after North Korea conducted a second nuclear test May 25 in defiance of the United Nations, Beijing and Moscow joined with the United States and other Security Council members in passing tough new sanctions.

Those measures include an expanded arms embargo, authorizing ship searches if there are reasonable grounds to suspect the vessels are carrying banned weapons or material to make nuclear weapons and ballistic missiles.