A dramatic statement from the Russian presidential administration claimed that Ukraine had used two drones to attack the Kremlin - at the heart of Moscow - overnight.

Russian forces apparently used radar equipment to disable the drones. There were no casualties, and the president was unhurt, it said.

But the Kremlin did call it "an assassination attempt" against Vladimir Putin.

A number of videos then surfaced. They appear to show at least one drone flying towards the Kremlin, followed by an explosion. Another appears to show smoke rising from a Kremlin structure, and a fire. The BBC has been unable to verify that it was a drone and it is unclear what really happened.

But if what the Kremlin is saying is true, and this was a genuine attempt on the president's life, then it would be a highly embarrassing incident for the Kremlin.

By all accounts, Mr Putin appears to be one of the most closely-guarded leaders in the world. At his events in Moscow attended by BBC journalists, extremely tight security has been in place, including extensive checks and long convoys of vehicles, with airspace closed and traffic halted.

Questions will now be asked about how well-protected the Russian leader is - and about the effectiveness of Russian air defences.

In recent months, anti-aircraft systems have been spotted on Moscow rooftops in the vicinity of key buildings, including the defence ministry. They have been placed there precisely for this reason - because the Kremlin is concerned that Ukraine, or those sympathetic to Ukraine, may attempt to carry out aerial attacks on high-value targets.

If that's what this incident was, then those measures failed.

This is the latest in a string of attacks and explosions that have hit Russian territory in recent weeks and months.

The last few days have seen a spike in such incidents. On Wednesday morning, a fuel depot caught fire in Russia's Krasnodar Region, reportedly caused by a drone attack. Two freight trains were derailed in Bryansk Region, near the border with Ukraine, on Monday and Tuesday, in separate but identical incidents. The local governor said improvised explosive devices were to blame.

Over the weekend, Russia-installed officials in occupied Crimea claimed Ukrainian drones were responsible for an attack on a fuel storage facility.

The frequency of such attacks is causing nervousness amongst ordinary Russians.

Russian media report that police in Moscow have been inundated with calls from the public reporting drone sightings in the capital.

Ukraine has vehemently denied attacking the Kremlin or targeting President Putin. But whether the Russian account of what happened is accurate or not, the question now is whether Moscow will respond - and if so, how.

Some Russian officials have already called for tough action. An assassination attempt against the president, if that's what this was, is an extremely serious matter. Russian generals have warned many times of harsh responses to strikes on Russian territory.

But does Russia have the capacity to carry out any meaningful retaliatory strikes? It remains to be seen whether this incident will lead to any significant escalation on the battlefield.

(editor-in-charge：Press center9)