The leaders of Russia and China urged North Korea on Wednesday to return to nuclear talks with the international community, saying they're concerned about recent threats from the reclusive state.

Chinese President Hu Jintao and Russia's Dmitry Medvedev, who met at the Kremlin, issued a joint statement saying they "expressed serious concern in connection with the situation on the Korean peninsula" and underscored the need for a peaceful resolution of the tension.

They called for the "swift" resumption of six-party talks, which include their nations, along with North and South Korea, the U.S. and Japan.

China and Russia have in the past used their status as veto-wielding permanent United Nations Security Council members to soften Western-backed sanctions against North Korea, but they approved new punitive measures this month after expressing unusually strong concern over North Korea's recent nuclear test and missile launches.

North Korea, which held its first nuclear test in 2006, conducted its second on May 25 in defiance of the United Nations. South Korean and Japanese news reports suggest Pyongyang is readying an additional test site for a missile that could reach the U.S.

Last week, the UN Security Council unanimously agreed to impose new sanctions on North Korea to reduce financial ties and extend a ban on exports of tanks, artillery and other large arms that represent a significant source of revenue for that country.

Earlier Wednesday, a top U.S. official told a meeting of the International Atomic Energy Association in Vienna that Washington "will not accept North Korea as a nuclear weapons state."

"We believe it is in North Korea's own best interests to return to serious negotiations," U.S. delegate Geoffrey Pyatt said.

North Korea responded by warning the U.S. and its allies of a "thousand-fold" military retaliation if provoked.

Pyongyang, which is believed to have enough weaponized plutonium for at least half a dozen atomic bombs, claims they are a deterrence against the United States and accuses Washington of plotting with South Korea to topple its government.

Washington is studying whether to put North Korea back on its list of state sponsors of terror after removing it last October.

If North Korea is added back to the list, not much would change for the reclusive state, because most of the sanctions required by the list are already in place due to other American laws.

The main sanctions include bans on arms and arms-related weapon sales and non-humanitarian aid to North Korea. Washington would also be prohibited from supporting any international loans to North Korea from institutions such as the World Bank.

With files from The Associated Press