Ukrainian forces have reported heavy Russian shelling and attempts to advance on several towns in the eastern region of Donetsk that has become a key focus of the near six-month war, but said they had repelled many of the attacks.
The General Staff of Ukraine's armed forces also reported Russian shelling of more than a dozen towns on the southern front - particularly the Kherson region, mainly controlled by Russian forces, but where Ukrainian troops are steadily capturing territory.
Much attention has been focused on the Zaporizhzhia nuclear plant in southern Ukraine amid fears of a catastrophe over renewed shelling in recent days that Russia and Ukraine blame on each other.
United Nations Secretary-General Antonio Guterres called for the establishment of a demilitarised zone and Ukrainian President Volodymyr Zelenskiy warned Russian soldiers who shoot at Europe's largest nuclear power station or use it as a base to shoot from that they will become a "special target" of Ukrainian forces.
The Zaporizhzhia plant dominates the south bank of a vast reservoir on the Dnipro River. Ukrainian forces controlling the towns and cities on the opposite bank have come under intense bombardment from the Russian-held side.
The International Atomic Energy Agency, which seeks to inspect the plant, has warned of a nuclear disaster unless fighting stops. Nuclear experts fear fighting might damage the plant's spent fuel pools or reactors.
Zelenskiy said Ukraine had many times proposed different formats to the Russian leadership for peace talks, without progress.
"So we have to defend ourselves, we have to answer every form of terror, every instance of shelling - the fierce shelling which does not let up for a single day," he said in video remarks late on Sunday.
Kyiv has said for weeks it is planning a counteroffensive to recapture Zaporizhzhia and neighbouring Kherson province, the largest part of the territory Russia seized after its February 24 invasion and still holds.
Ukraine's military command on Sunday said Russian soldiers had continued unsuccessfully to attack Ukrainian positions near Avdiivka, which, since 2014, has become one of the outposts of Ukrainian forces near Donetsk.
Reuters could not independently verify the battlefield accounts.
Russia calls its invasion of Ukraine a "special military operation" to demilitarise and "denazify" its smaller neighbour. The war has pushed Moscow-Washington relations to a low point, with Russia warning it may sever ties.
While Russia has been largely isolated on the global diplomatic stage, North Korean state media on Monday said Russian President Vladimir Putin told leader Kim Jong-un the two countries would expand "comprehensive and constructive" ties.
In July, North Korea recognised as independent states the Russian-backed breakaway "people's republics" of Donetsk and Luhansk, and officials raised the prospect of its workers being sent there to help in construction and other labour.
Ukraine immediately severed ties with Pyongyang over the move.z)