ReutersUkraine's military has reported Russian drone attacks on several regions overnight, just hours after the end of a 30-hour "Easter truce" declared by Moscow.
Air raid alerts were issued by Ukraine's air force for the Kyiv region, as well as Kherson, Dnipropetrovsk, Cherkasy, Mykolaiv and Zaporizhzhia.
In the southern city of Mykolaiv, Mayor Oleksandr Senkevych said "explosions were heard". It was not immediately clear whether there were any casualties.
Russia's defence ministry confirmed it has resumed fighting, adding that its military had "strictly observed the ceasefire and remained at the previously occupied lines and positions".
The truce declared by President Vladimir Putin expired at midnight on Sunday Moscow time (21:00 GMT). Both sides accused each other of violating the ceasefire thousands of times.
Early on Monday residents in several Ukrainian cities, including the capital Kyiv, were urged by local authorities to go immediately to nearby shelters due to the threat of drone strikes.
In the Kyiv region, local officials said air defence forces were "working on targets".
Ukraine's air force also reported a "rocket danger" for central regions, and said Russian aircraft were "active in the north-eastern and eastern directions".
In an update on Telegram, the air force said Russia launched 96 drones overnight, as well as striking the southern region of Mykolaiv two missiles and Kherson with a third missile.
In Mykolaiv, regional head Vitaliy Kim said shortly afterwards that the city had been attacked by missiles. "There were no casualties or damage," he added.
Several hours before the truce expired, Kremlin spokesman Dmitry Peskov said Putin had not given an order to extend it, Russia's state-run news agency Tass reported.
The BBC has not independently verified the claims by Ukraine and Russia.
US President Donald Trump - who has been pushing for an end to the war - said late on Sunday that "hopefully Russia and Ukraine will make a deal this week". He gave no further details.
Russia launched a full-scale invasion of Ukraine on 24 February 2022, and currently controls about 20% Ukraine's territory, including the southern Crimea peninsula annexed by Moscow in 2014.
It is estimated that hundreds of thousands of people - the vast majority of them soldiers - have been killed or injured on all sides since 2022.
Last month, Moscow came up with a long list of conditions in response to a full and unconditional ceasefire that had been agreed by the US and Ukraine.
On Saturday, President Putin said there would be an end to all hostilities from 18:00 Moscow time (15:00 GMT) on Saturday until midnight on Sunday. Kyiv said it would also adhere.
"For this period, I order all military actions to cease," Putin said in his announcement.
"We assume that the Ukrainian side will follow our example. At the same time, our troops must be ready to repel possible violations of the truce and provocations from the enemy, any of its aggressive actions."
However, Ukraine's President Volodymyr Zelensky said late on Sunday there had been a total of 1,882 cases of Russian shelling, 812 of which involved heavy weaponry, according to a report from Commander-in-Chief of the Armed Forces of Ukraine Oleksandr Syrskyi.
The president said the heaviest shelling and assaults were in eastern Ukraine near the besieged city of Pokrovsk, a key logistics hub in the Donetsk region.
"The nature of Ukrainian actions will continue to be mirrored: we will respond to silence with silence, our strikes will be to protect against Russian strikes," Zelensky said.
Earlier on Sunday, he said "there were no air raid alerts today", referring to Russia's daily drone and missile strikes against Ukraine.
He proposed "to cease any strikes using long-range drones and missiles on civilian infrastructure for a period of at least 30 days, with the possibility of extension".
Zelensky also said Putin's declaration of a truce amounted to a "PR" exercise and his words were "empty". He accused the Kremlin of trying to create "a general impression of a ceasefire".
"This Easter has clearly demonstrated that the only source of this war, and the reason it drags on, is Russia," the president said.
The Russian defence ministry insisted its troops had "strictly observed the ceasefire".
Meanwhile, Russian Foreign Ministry spokeswoman Maria Zakharova accused Ukraine of using US-supplied Himars missiles during the ceasefire.
The surprise ceasefire announcement came shortly after Trump threatened to "take a pass" on brokering further Russia-Ukraine peace talks.
However, a state department spokesperson said on Sunday Washington remained "committed to achieving a full and comprehensive ceasefire".
"It is long past time to stop the death and destruction and end this war," the spokesperson added.
