Germany has accused Russia of ordering the murder of a man killed in a Berlin park last August.
Federal prosecutors accused "government agencies of the central government of the Russian Federation" of ordering the killing.
A Russian national identified as Vadim K has been charged with the murder.
Russia has previously called accusations it was involved in the crime "absolutely groundless" and said it had nothing to do with the death.
Zelimkhan Khangoshvili - who lived for a time under the name Tornike Kavtarashvili - was shot dead in broad daylight last August in Berlin's Kleiner Tiergarten park. The 40-year-old Georgian national was a former Chechen rebel commander.
German foreign minister Heiko Maas told reporters the Russian ambassador had been called in for talks, adding that the federal government "explicitly reserves the right to take further measures".
Russia's ambassador in Berlin, Sergei Nechayev, later said the prosecutors' accusation was "not supported by any facts or evidence".
"We consider the allegations issued against Russian state structures as groundless and unfounded," Mr Nechayev said in a post on the embassy's Facebook page.
He added that Moscow would respond without fail to any further steps taken by Germany.
Media have compared the attack on Khangoshvili to the attempted murder of Russian former intelligence agent Sergei Skripal and his daughter in the UK in 2018.
What's the accusation?
Germany's federal prosecutor issued a statement on Thursday announcing charges against a Russian citizen Vadim K, otherwise known as Vadim S.
He took the "government order to kill", it reads, in the hope of financial reward or because he shared the desire to "kill a political opponent".
Prosecutors say in August 2019 Vadim K flew from Moscow to Paris and from there to Warsaw, before arriving in Berlin, using a passport under the name Vadim S issued by Russian authorities just a month previously.
On 23 August he allegedly came up behind Khangoshvili on a bicycle and fired a shot into his torso using a silenced Glock 26 pistol. Prosecutors say he then fired two more shots into the victim's head as he lay on the ground, killing him.
Vadim K was arrested shortly afterwards and has been in pre-trial detention since.
Who was the victim?
The victim Tornike K was a Georgian citizen of Chechen descent and had been an asylum seeker in Germany since 2016, the statement said.
He had fought against Russia as the leader of a Chechen militia between 2000 and 2004, and also put together a group of volunteers to defend the enclave of South Ossetia in 2008 on behalf of the Georgian government - although they were never deployed.
"Russian authorities also classified Tornike K as a terrorist and accused him of being a member of the terrorist association 'Caucasian Emirate'," according to the statement.
In December Germany expelled two Russian diplomats due to suspicions the Kremlin was involved in the murder. Russia responded by expelling two German diplomats.