Moscow ? Russia and China urged North Korea on Wednesday 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 U.S. and its allies if provoked. The United States, meanwhile, called on Pyongyang to stop its saber-rattling and negotiate.

The fact that the 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 both border North Korea and Pyongyang’s unpredictable actions have raised concern in both countries.