At least seven people have been killed including two children during intense Russian drone and missile strikes on Ukraine, officials say.

A kindergarten was hit in Ukraine's second biggest city Kharkiv and there was widespread damage in Kyiv. Children were among the 27 people wounded.

Hours earlier, US President Donald Trump said his plans for an imminent summit in Budapest with Russia's Vladimir Putin had been shelved as he did not want a wasted meeting.

President Volodymyr Zelensky said the attacks proved Moscow had not come under enough pressure for its continued war.

The Kremlin has rejected calls for a ceasefire along the current front lines made both by Trump and European leaders.

However, Russian officials said on Wednesday that preparation for a Trump-Putin summit was continuing, disputing the US president's assessment that it had been put on hold.

Kremlin spokesman Dmitry Peskov said a date had yet to be agreed upon and careful preparation was needed. He said a summit was the mutual desire of both presidents and most of the gossip and rumours surrounding it were untrue.

Ukraine's president arrived in Norway on Wednesday at the start of a European trip, days after talks with Trump in Washington last Friday, when he failed to persuade the US president to provide long-range Tomahawk missiles.

Zelensky told reporters in Oslo that Trump's proposal to freeze the front line was a good compromise, but I'm not sure that Putin will support it and I said it to the president.

The Ukrainian leader has directly linked the shelving of the planned Trump-Putin summit to his request for US Tomahawks: As soon as the issue of long-range missiles became a little further away for us, for Ukraine, then almost automatically Russia became less interested in diplomacy.

Following a meeting on Wednesday, Zelensky and Swedish Prime Minister Ulf Kristersson signed a letter of intent that could see Sweden supply 100 to 150 domestically produced Gripen fighter jets to Ukraine.

The jets are expected to be delivered over a 10-15 year timeline, with the first sent to Ukraine within three years, Kristersson added.

Russia's drone and missile bombardment came shortly after Ukraine's military said it had attacked a Russian chemical plant in the Bryansk border region late on Tuesday with UK-supplied Storm Shadow missiles.

Calling the strike a successful hit that penetrated the Russian air defence system, military officials said the Bryansk plant produces gunpowder, explosives and rocket fuel components used in ammunition and missiles employed by the enemy to shell the territory of Ukraine.

The attacks on Ukraine persisted, resulting in several casualties, including children, and casting shadow over ongoing diplomatic efforts aimed at ceasing hostilities.