UN Secretary-General Antonio Guterres has expressed concern that the war in Ukraine may be prolonged.

Guterres was in Japan to attend Saturday's ceremony in Hiroshima to mark the 77th anniversary of the atomic bombing of the western Japan city. He met reporters in Tokyo on Monday.

He said Ukraine cannot accept another country taking its territory. He said Russia does not seem ready to accept that it cannot annex areas it has taken and that they won't become new independent states.

Guterres said he is very worried that there might be a prolonged war as the positions of Russia and Ukraine are currently impossible to reconcile.

He also said the UN has a very strong humanitarian operation in Ukraine and that the UN is ready to mediate on any practical issue created by the conflict.

When questioned on rising tensions in the Taiwan Strait, with China's large-scale drills being held near Taiwan, Guterres said he naturally views the events with concern.

He said, "The United Nations abides by the resolution of the General Assembly, with the so-called one China policy, but we all want that resolution to correspond to a peaceful environment." He added that the situation needs to be treated with common sense and restraint.

Guterres, touching on climate change, also expressed unease about Japan and some other countries continuing to provide loans for coal-fired power generation.

He said he hopes the Japanese public and private capital will stop financing coal completely. He added he is counting on Japan to quickly deliver on its pledge to reduce its reliance on coal and to push beyond that.

He said, "There is no such thing as clean coal."