Russia has pledged humanitarian assistance to North Korea after devastating floods damaged thousands of homes and caused an unknown number of casualties, with reports from South Korea that the number of dead or missing could be as high as 1,500.

President Vladimir Putin offered condolences and humanitarian aid after a record downpour on 27 July which submerged swathes of farmland in the north near China, the Kremlin and North Korean state media said.

“I ask you to convey words of sympathy and support to all those who lost their loved ones as a result of the storm,” Putin said in a telegram to Kim, adding “you can always count on our help and support”.

North Korean leader Kim Jong-un thanked Putin for the offer but said since his government has already taken measures to conduct recovery work, he would ask for help “if aid is necessary”, KCNA state media said.

Heavy rains have pummelled North Korea’s north-western areas in recent days, flooding more than 4,000 homes and isolating 5,000 residents, KCNA has reported. State media said Kim had personally inspected the affected areas.

On Saturday, Kim accused South Korean media outlets of spreading rumours about damage and casualties from the floods, days after Seoul reached out to offer humanitarian aid.

South Korea’s government on Thursday said it was willing to “urgently provide” humanitarian assistance to “North Korean disaster victims” following reports in local media that the toll of dead and missing could number 1,500.

The report by South Korea’s TV Chosun, which was later picked up by other outlets, also reported on the possible death of rescue workers killed in helicopter crashes.

North Korea’s Kim slammed the reports for “spreading the false rumour that the human loss … is expected be over 1,000 or 1,500”, according to Pyongyang’s Korean Central News Agency.

The flooding reports constituted a South Korean “smear campaign to bring disgrace upon us and tarnish” the North’s image, he added.

North Korea and Russia have been allies since the North’s founding after the second world war, but Pyongyang and Moscow have ramped up diplomatic and security ties in recent months, with Kim and Putin exchanging visits and signing a “comprehensive strategic partnership” pact in June.

Reuters and Agence-France Presse contributed to this report.