KIEV (Reuters) - U.S. Secretary of State John Kerry said on Thursday he hoped China would continue to cooperate with the United States on sanctions on North Korea, a day after Beijing criticized new U.S. sanctions targeting North Korean leader Kim Jong Un.

"China's engagement is critical," Kerry told a news conference during a visit to the Ukrainian capital Kiev, when asked if sanctions could be effective without China's help.

Kerry said he spoke to China's foreign minister Wang Yi on Wednesday about cooperation on North Korea.

"Our hope is that we continue to cooperate as we have been in the last months, particularly with the U.N. Security Council resolution that we passed in which China stepped up and significantly increased its own actions with respect to (North Korea)," he said.

At the same time, Kerry said the United States stood "ready and prepared" to return to talks with North Korea aimed at convincing the country to abandon its nuclear weapons program.

China's foreign ministry said earlier on Thursday it opposed the use of unilateral sanctions after the United States announced sanctions on the North Korean leader for the first time.

China has signed up for tough United Nations sanctions against nuclear-armed North Korea, but has opposed past unilateral sanctions imposed by the United States.

Washington announced sanctions on Kim for the first time on Wednesday, citing "notorious abuses of human rights," a move diplomats say will infuriate Pyongyang, where the leader is considered infallible.

Some analysts and diplomats said before the U.S. announcement that such a move could limit cooperation with China on further international sanctions aimed at rolling back North Korea's nuclear weapons program.

China argues that the human rights situation in North Korea is not a threat to international peace and security and has sought to prevent the issue being discussed at the U.N. Security Council.

(Reporting by Lesley Wroughton in Kiev and David Brunnstrom in Washington; Editing by James Dalgleish)