President Volodymyr Zelensky has said that his forces are "in full control" in the Sumy region in the northeast of Ukraine, and dismissed as "propaganda" a Russian claim to have taken a village on the border.
Mr Zelensky said that Russian sabotage groups had been "destroyed".
"As for the village of Ryzhivka, the occupier tried to implement a propaganda operation there.
"As of this morning, the Russian flag in the village was destroyed, and there is no occupant presence," the Ukrainian leader said in a statement on social media.
The leader of Russia's Chechnya region claimed yesterday that a special forces unit had led an attack which seized control of the area. Ramzan Kadyrov said that the Akhmat-Chechnya unit spearheaded Russian troops in taking control of Ryzhivka.
The "large-scale planned advance" inflicted "significant losses on the Ukrainian side, which was forced to retreat," Mr Kadyrov claimed.
Andriy Kovalenko, head of the Ukrainian government's Centre for Countering Disinformation, said on Telegram that Russian forces had tried to test Ukraine's defences in the area.
He insisted that Ukrainian forces remain in control, but emphasised the ongoing threat posed by Russian forces in the border area.
Russia's Defence Ministry has made no comment on the reported advance.
It did, however, issue a statement claiming to have taken Staromaiorske, in the southeast of Ukraine.
In recent weeks, Ukraine's military has warned of a buildup of Russian forces around the Sumy region, where a big push would open a new front in the war and threaten to over-extend Ukrainian forces.
On 10 May, Russia mounted a major ground operation in the neighbouring Kharkiv region, capturing several towns and villages as Ukrainian forces struggled with manpower and ammunition shortages.
In May, Ramzan Kadyrov told Russian President Vladimir Putin that tens of thousands of his Chechen soldiers were prepared to fight in Ukraine. He said that 43,500 troops had already served in Moscow's war.
'Legitimate targets'
State-owned media in Russia has quoted a senior politician as having claimed that any F-16 aircraft, along with their bases outside Ukraine, will be legitimate targets if they are used in combat missions.
Andrei Kartapolov is the head of the defence committee of the Duma, Russia's parliament.
His remarks, cited by the RIA news agency, came as Ukraine's general staff said that it had hit three Russian surface-to-air defence systems in Russian-occupied Crimea overnight.
It said that "successful strikes" were launched at an S-400 system in Dzhankoi, and two S-300 systems near Yevpatoriya and Chornomorske.
An immediate shutdown of the systems' radars was recorded after the strikes, as well as detonation of ammunition in the area, a Ukrainian statement on Telegram said.
Meanwhile, Belarus has confirmed that its military is taking part in the second stage of Russian exercises ordered by Vladimir Putin to practise the deployment of tactical nuclear weapons.
The first phase of the drills took place in southern Russia last month, in what nuclear analysts said was a warning from Mr Putin to deter the west from wading more deeply into the war in Ukraine.
Belarusian Defence Minister Lieutenant General Viktor Khrenin claimed that the exercises were a proactive measure to "increase our readiness to use so-called retaliatory weapons".
Peace conference
NATO Secretary General Jens Stoltenberg will visit Budapest this week, Hungarian Prime Minister Viktor Orban has told state media.
"In two days time the secretary general will be here, to discuss Hungary's staying out from NATO's Ukraine mission," Mr Orban said of Wednesday's visit.
Switzerland will stage a conference on the conflict this weekend, with world leaders set to discuss how to mount an eventual peace process.
Russia is not taking part in the conference, which will take place immediately after a G7 summit in southern Italy, which runs from Thursday to Saturday, and which will be attended by Ukrainian president, Volodymyr Zelensky.
The G7 will look at ways to use frozen Russian assets to provide fresh aid to Ukraine, which was invaded by Russia in February 2022.
Mr Zelensky will then head to Switzerland, to be joined by G7 and other leaders for the summit, which is being billed as the first, "Summit on Peace in Ukraine", and will take place at the luxury Burgenstock resort on a mountain ridge overlooking Lake Lucerne.
French President Emmanuel Macron, US Vice President Kamala Harris, German Chancellor Olaf Scholz and Japanese Prime Minister Fumio Kishida are among those so far confirmed to be attending.
The aim "is to inspire a future peace process and to develop practical elements as well as steps towards such a process", the Swiss foreign ministry said.
