Russian President Vladimir Putin has instructed his troops to hold nuclear weapons drills near Ukraine in a show of force to western leaders.
Moscow said the exercises featuring short-range tactical nuclear arms would be held in the "near future" and involve ground, air and naval forces stationed near Ukraine.
The Kremlin said the "special measures" were prompted by western hints that Nato troops could be deployed on the battlefield in Ukraine.
French President Emmanuel Macron has repeatedly said the West cannot rule out sending in ground troops, breaking what has been a taboo.
"It's obvious we are talking about statements from Mr Macron and from British representatives," Kremlin spokesman Dmitry Peskov said.
"They are talking about the readiness and even the intention of sending armed contingents to Ukraine ... that is, in fact, to put Nato soldiers in front of the Russian military."
"This is a completely new round of escalating tensions. It is unprecedented and requires special measures."
Ukraine's military intelligence agency condemned the planned drills.
"We do not see here anything new ... nuclear blackmail is a constant practice of Putin's regime," spokesman Andriy Yusov told national television.
Mr Putin has repeatedly hinted Russia could use nuclear weapons in Ukraine, drawing alarm from UN officials and foreign leaders.
Moscow's Defence Ministry said the drills were aimed at ensuring Russian territorial integrity in the face of "threats by certain western officials".
Troops from Russia's southern military district, which borders Ukraine and includes occupied Ukrainian territories, will take part in the drills, the ministry said.
"During the exercise, a set of measures will be taken to practise the preparation and use of non-strategic nuclear weapons," it announced.
Tactical nuclear weapons have a lower yield compared to massive warheads that arm strategic ballistic missiles intended to obliterate entire cities.
The announcement came as authorities in Russia's Belgorod region, which borders Ukraine, reported the deadliest attack in weeks with six people reported killed.
Belgorod governor Vyacheslav Gladkov said two moving vans and a car in the region came under attack by Ukrainian kamikaze drones.
The Belgorod region was a staging ground for Russia's invasion of Ukraine in February 2022 and has come under regular attack since troops retreated there.
Britain's Foreign Secretary David Cameron last week said Ukraine's forces could use UK-provided weapons to strike the Russian mainland, further angering the Kremlin.
Ukraine, meanwhile, said overnight Russian strikes had targeted energy facilities in the northern Sumy region and north-eastern Kharkiv region – both of which have come under an increasing number of recent attacks.
Thousands of homes were left without power in the aftermath of the strikes, the Ukrainian Energy Ministry said.
