Ukrainian President Volodymyr Zelensky has suggested that Russia was "preparing something" in Belarus this summer, using military drills as an excuse.
"This summer Russia is preparing something there, under cover of military exercises," Mr Zelensky told a summit in Poland, without giving details or citing evidence.
Russia and Belarus have announced joint military drills for September.
Meanwhile, Russian troops are trying to carve out a buffer zone in Ukraine's northeastern Sumy region, although so far without substantial success, the regional governor has said.
The Sumy region borders on Russia's Kursk region, where Russian forces have lately recaptured territory seized last year in the Ukrainian military's only major cross-border incursion of the war.
Ukraine has said for months that it is concerned about Russia's troops trying to advance into Sumy.
Russia has said its forces have captured some territory there and have driven all Ukrainian troops out of Kursk.
'Grey zone'
Sumy Governor Oleh Hryhorov said on the Telegram app that four border villages in Sumy - Zhuravka, Veselivka, Basivka and Novenke - were in a "grey zone" due to Russian attacks, but were not under Russian control.
"Currently, the enemy continues to try to carve out a buffer zone on the territory of our region, but has not had any significant success," Mr Hryhorov said.
Of the four villages, he said "there is no question of their occupation at the moment".
The Russian defence ministry said its troops had captured the village of Doroshivka in Ukraine's northeastern Kharkiv region, thought this could not be independently verified.
Russia's defence ministry said earlier in April that its forces had captured Zhuravka and Basivka, which Ukraine denied.
DeepState, an open source map compiled from observations of the battlefield, says about 52 square kilometres of the Sumy region are in a contested grey zone.
The Russian movement in Sumy comes after Russian and Ukrainian forces fired dozens of drones at each other early this morning, killing a 12-year-old girl in Ukraine and two people in a Russian border region, authorities said.
The attacks came less than two days into a week the United States warned would be "critical" for the peace process and as US officials threaten to abandon their push for a ceasefire if they do not see progress soon.
Russian strikes on Ukraine have killed dozens of people over the past month, including in the capital Kyiv, while hostilities on the front line have shown no signs of abating.
Yesterday, Russian President Vladimir Putin declared a three-day truce in May to coincide with Moscow's World War II commemorations.
Mr Zelensky said this was only intended for Mr Putin to "have silence" during Moscow's 9 May military parade and demanded an immediate ceasefire.
Russia fired 100 drones at Ukraine between late yesterday and early this morning, the Ukrainian air force said.
"The enemy attack led to a tragedy. In Gubynykha, in the Samar district, a 12-year-old girl died," the governor of Ukraine's central Dnipropetrovsk region, Sergiy Lysak, wrote on Telegram.
Gubynykha is around 140 kilometres (90 miles) west of the front line.
"A 6-year-old girl and two adults were injured," he added.
In Russia's Belgorod region, which borders Ukraine, "an enemy drone deliberately struck a moving vehicle carrying five men", regional governor Vyacheslav Gladkov said.
"To our great sorrow, two people died at the scene before the arrival of medical personnel," he added.
The Russian army said it shot down 40 drones over various regions overnight, including four over the Russian-controlled Crimean peninsula.
Since launching its Ukraine offensive in February 2022, Russia has seized large parts of four Ukrainian regions and claimed them as its own, in addition to Crimea, which it annexed in 2014.
