Russia has blamed the US for an attack on a pro-Kremlin writer that killed his assistant and left him wounded.
The head of the Russian paramilitary group Wagner meanwhile asked Moscow to let Chechen fighters relieve his forces at the flashpoint city of Bakmut in eastern Ukraine.
Earlier, investigators said Ukraine was behind the blast that wrecked the car in which Zakhar Prilepin, one of Russia's best-known novelists, was travelling.
It took place in Mr Prilepin's region of Nizhny Novgorod, some 400km east of Moscow.
The investigative committee published images of a partly destroyed, overturned car and said the writer had been taken to a medical facility, naming a suspect they had detained as Alexander Permyakov.
Mr Permyakov said he had acted "on the instructions from the Ukrainian special services", said Russia's investigative committee, which handles major crimes.
An interior ministry video showed the suspect in handcuffs, wearing a khaki cap and a black hoodie.
But Ukrainian presidential aide Mykhaylo Podolyak appeared to suggest the attack was due to Russian in-fighting, referring to the biblical figure Moloch, a pagan deity who "devours his enemies... and finally devours his own".
Later, Russia's foreign ministry said: "The responsibility for this terrorist act, and for others, does not lie only with Ukraine, but also with its Western minders, primarily the United States."
It denounced the lack of condemnation of the attack from Washington and said that "the silence of the relevant international is unacceptable".
Regional governor Gleb Nikitin said he visited Prilepin in hospital and "the operation was successful".
Mr Prilepin is a vocal supporter of Moscow's offensive in Ukraine, where he fought alongside pro-Russian separatists in 2014.
His novels draw on his experiences of serving with Russian forces in Chechnya and as a member of a banned radical nationalist group.
The shaven-headed writer has been a frequent visitor to pro-Russian separatists in eastern Ukraine since the start of the conflict in April 2014.
There have been two previous killings of nationalists, both of which Russia blamed on Ukraine.
In April, a blast from a statuette rigged with explosives killed 40-year-old pro-Kremlin military blogger Vladlen Tatarsky.
The Kremlin said that attack had been orchestrated by Ukraine with the help of supporters of jailed Kremlin critic Alexei Navalny.
But observers said the April bombing could be used to justify a further crackdown on critics.
And last August Darya Dugina, the daughter of a prominent ultranationalist intellectual, was killed in a car bombing outside Moscow, which Russia blamed on Ukraine. Kyiv denied the charges.
Wagner chief's appeal
The chief of the Russian paramilitary group Wagner meanwhile asked Moscow to let him hand over his positions in the eastern Ukrainian city of Bakhmut to Chechen leader Ramzan Kadyrov's forces.
"I ask you to issue a combat order before 00:00 on May 10 concerning the transfer of the positions of the Wagner paramilitary units in Bakhmut and its periphery, to the units of the Akhmat battalion," Yevgeny Prigozhin said in a letter to Defence Minister Sergei Shoigu.
The Akhmat battalion refers to combat units under the command of strongman Kadyrov, who has ruled Russia's Muslim-majority republic Chechnya for the last decade and a half.
Wagner fighters have led the battle for Bakhmut, spearheading the grinding, months-long Russian assault on the city and almost capturing it in what has been the longest and bloodiest battle of the Russian campaign in Ukraine.
But rivalries between Prigozhin and the Russian army, which have long been strained, reached boiling point this week.
In a series of scathing videos on Friday, Mr Prigozhin blamed Mr Shoigu and Chief of General Staff Valery Gerasimov for "tens of thousands" of killed and wounded Russian fighters in Ukraine.
Mr Prigozhin said his fighters would be forced to pull out because of a lack of ammunition, blaming the defence ministry.
Mr Kadyrov said on Telegram Friday that his forces were "ready to move" towards Bakhmut.
"The soldiers are on alert, we are just waiting for orders," he said.
Increasing sabotage
As Russia prepares to celebrate the popular 9 May celebrations of the Soviet victory over the Nazis, Moscow has blamed Kyiv, and its Western supporters, for a series of attacks and sabotage operations.
In the most spectacular incident, Russian authorities claimed Thursday to have thwarted a drone attack on the Kremlin, blaming Washington for having masterminded the attack.
It said the attack, by two drones, had been aimed at killing Russian President Vladimir Putin. Both Kyiv and Washington have denied any involvement.
The authorities in Crimea, annexed by Russia in 2014, and in Russia's southern regions of Krasnodar and Rostov, both near Ukraine, have all reported drone strikes -- or attempted drone attacks, in recent days.
On the Ukrainian side, Kyiv said Russian fire had killed six Ukrainian emergency workers demining in the southern region of Kherson.
