A Ukrainian court ruled a journalist from Russia's state-run RIA Novosti news agency, who is accused of treason, should be held in detention until his trial.
The Kherson city court in southern Ukraine yesterday said Kyrylo Vyshynsky will be kept behind bars until at least July 13, at the request of the prosecutor's office.
Vyshynsky, a Ukrainian national who received a Russian passport in 2015, was on Tuesday detained near his house in Kiev as Ukraine's SBU security service raided RIA Novosti's offices there.
The case has sparked a fresh row between Kiev and Moscow, with Russia on Wednesday saying it would do "everything to ensure its citizen is freed".
Vyshynsky denies the charges against him and his lawyer Andriy Domansky told journalists he would file an appeal against Thursday's ruling.
Moscow yesterday promised "retaliatory measures" condemning "repressive acts towards a journalist who has always carried out his professional work honestly and conscientiously".
More From This Section
Both the EU and United States have said that while Ukraine has the right to protect its national security, it must respect human and legal rights.
The the Organisation for Security and Co-operation in Europe (OSCE) and the Council of Europe have expressed concern at the detention of Vyshynsky.
SBU accused Vyshynsky of travelling to Crimea in 2014 to carry out "subversive" reporting to justify the peninsula's annexation by Russia.
The 51-year-old was also accused of collaborating with Russian-backed separatist groups in eastern Ukraine where a conflict that broke out following Crimea's seizure has cost some 10,000 lives.
An SBU spokeswoman Olena Gitlyanska said Russian-controlled media were being "used as tools in a hybrid war against Ukraine".
Kiev and its Western allies regularly accuse Russia of using its public media for propaganda and incitement of separatist sentiment in Ukraine.
Ukraine ranks 101st out of 180 countries in the 2018 World Press Freedom Index, compiled by Reporters Without Borders (RSF) as Russia came in at 148.