Ukrainian President Volodymyr Zelenskiy has appealed to leaders of the G7 for more air defence capabilities as the group vowed to support Ukraine for "as long as it takes" while warning Russia against any use of nuclear weapons.
The NATO military alliance said it was closely monitoring Russia's nuclear forces following a string of Russian battlefield defeats in Ukraine and that the allies were also boosting security around key infrastructure after recent attacks on Baltic Sea gas pipelines.
Russian missiles again hit Ukrainian cities on Tuesday but with less intensity than on Monday, when dozens of strikes killed 19 people, wounded more than 100 and knocked out power supplies across the country in the biggest aerial offensive since the start of Russia's invasion on February 24.
More missile strikes killed at least one person in the southeastern Ukrainian town of Zaporizhzhia and left part of the western city of Lviv without power, local officials said.
Russia said it continued to launch long-range air strikes on Ukraine's energy and military infrastructure on Tuesday.
Air raid sirens earlier wailed across Ukraine for a second day.
"When Ukraine receives a sufficient quantity of modern and effective air defence systems, the key element of Russia's terror - rocket strikes - will cease to work," Zelenskiy told G7 leaders at a virtual meeting where he again ruled out peace talks with Russian President Vladimir Putin.
Ukraine received the first of four IRIS-T air defence systems Germany promised to supply, a German defence ministry source said.
The White House later said the United States was speeding up the shipment of sophisticated NASAMS air defences to Ukraine.
The US has provided more than $US16.8 billion ($A26.5 billion) worth of security aid to Ukraine during the war.
The G7 - which groups the United States, Germany, France, Japan, the United Kingdom, Italy and Canada - pledged continued "financial, humanitarian, military, diplomatic and legal support... for as long as it takes" to Ukraine, it said in a statement.
It also condemned "indiscriminate attacks on innocent civilian populations" as war crimes and said Putin would be held to account for them.
Russia, which calls its actions in Ukraine a "special military operation" to eliminate dangerous nationalists and protect Russian-speakers, has accused the US and its allies of escalating and prolonging the conflict by supporting Ukraine.
"We warn and hope that they realise the danger of uncontrolled escalation in Washington and other Western capitals," Russian Deputy Foreign Minister Sergei Ryabkov was quoted as saying by RIA news agency on Tuesday.
Ukraine and its foreign backers accuse Russia of an unprovoked imperialist land grab in Ukraine.
Belarus said it had begun an exercise to assess its "combat readiness" after ordering troops on Monday to deploy with Russian forces near its border with Ukraine.
Belarus allowed Russia to use its territory to invade Ukraine but has not sent its own troops across the border.
Zelenskiy denied Belarus officials' claims that Ukraine planned to attack Belarus but told the G7 he wanted to make sure there was no threat from its northern neighbour, and he called for a mission of international observers to monitor the border area.
Putin, under domestic pressure to ramp up the war as his forces have lost ground since early September, said he ordered Monday's strikes as revenge for an explosion that damaged Russia's bridge to annexed Crimea last weekend.
Residents in the capital Kyiv took cover on Tuesday for a second day deep in the underground metro, where trains were still running.
NATO has not noticed any change in Russia's nuclear posture following threats, Secretary General Jens Stoltenberg told reporters in Brussels on Tuesday.
The alliance will proceed with its annual nuclear preparedness exercise "Steadfast Noon" next week - in which NATO air forces practise the use of US nuclear bombs based in Europe with training flights - without live weapons.
The governor of Russia's Belgorod region said on Tuesday that more than 2000 people had been left without power after Ukraine shelled an electricity substation in the town of Shebekino, on the border with Ukraine's Kharkiv region.
© DPA 2022