BEIJING (Reuters) - The leaders of China and North Korea will meet when it is convenient for them, China's foreign minister said on Sunday, calling China's isolated neighbouring country a friend.

North Korean leader Kim Jong Un has yet to visit China, his country's only remaining ally of any importance, since assuming office after his father died in 2011, though he is expected to go to Russia this year.

Ties with China have cooled since Mr Kim took over and then, in 2013, defied international warnings and United Nations sanctions to conduct a third nuclear test. "As to when leaders of the two countries will meet, we have to see when it is convenient for both parties," Chinese Foreign Minister Wang Yi told reporters on the sidelines of an annual meeting of China's Parliament.

Mr Wang described North Korea as a "friendly neighbour". The two were allies in the 1950-1953 Korean War. "We cherish our traditional friendship with North Korea and we seek the normal development of our relations. The China-North Korea relationship has a strong foundation... It should not and will not be affected by temporary events."

Chinese President Xi Jinping has met South Korea's President Park Geun Hye six times since both took office in 2012, underscoring strengthening ties.

North Korea has conducted three nuclear tests, the last in February 2013, and is under UN sanctions for defying international warnings not to set off atomic devices in pursuit of a nuclear arsenal.

It often promises to call off nuclear and missile tests in return for steps by Washington to ease tension. It reached a deal in February 2012 with the United States for an arms tests moratorium only to scrap it two months later.

Last week, North Korean Foreign Minister Ri Su Yong said military exercises being staged by South Korea and the United States were "unprecedentedly provocative".

Mr Wang said that it was in the interests of all parties to maintain peace and stability on the Korean Peninsula. "At the moment the situation there has entered into another delicate period. We call on the relevant countries to exercise calm and restraint and say and do things that will have a positive effect so as to continue fostering an atmosphere and conditions for resuming the six-party talks."

China has repeatedly urged North Korea to return to so-called six-party talks that aimed to get the North to halt its nuclear programme.

In 2009, North Korea said it would never return to the talks which include South Korea, the United States, Japan and Russia.