Russian President Vladimir Putin has accused Ukraine of carrying out a border raid in Crimea that allegedly killed two Russian servicemen.
An allegation the Ukrainian government has denied.
Russia annexed Crimea two years ago and now controls the territory. It has recently increased its troop numbers along Crimea's border with Ukraine.
Kiev says it will respond in kind.
Could the worst escalation of tensions in months re-ignite the wider conflict between the two neighbours? Or is it just a case of sabre rattling?
Presenter: Folly Bah Thibault
Nataliya Gumenyuk - head of the Ukrainian news channel Hromadske TV
Sergey Strokan - political commentator at the Russian daily newspaper Kommersant
Domitilla Sagramoso - lecturer in the department of war studies at King's College London
Source: Al Jazeera