Ukraine's President Volodymyr Zelensky has denied his country carried out an alleged drone attack on the Kremlin, which Russia says was an attempt on President Vladimir Putin's life.
"We don't attack Putin or Moscow. We fight on on our territory. We are defending our villages and cities," he said, speaking on a visit to Finland.
The Russian president's office said defences downed two drones overnight.
It threatened to retaliate when and where it considered necessary.
Unverified footage circulating online shows smoke rising over the Kremlin - a large government complex in central Moscow - early on Wednesday. A second video shows a small explosion above the site's Senate building, while two men appear to clamber up the dome.
The Russian presidency said Ukraine had attempted a strike on Putin's residence in the Kremlin and described it as "a planned terrorist act and an assassination attempt on the president".
Officials said two drones targeting the complex had been disabled using electronic radar assets, adding that President Putin had not been in the complex at the time of the alleged attack.
But Ukraine has said the Russian accusations are merely a pretext for massive attacks on its territory and the US says it is treating the Russian claims with a lot of caution.
People on the dome of the Senate Palace, one of the main buildings within the Kremlin compound, on 3 May 2023 after Russia said it had shot down two drones aimed at President Vladimir Putin's Kremlin residence in what it called a Ukrainian assassination attempt. Photo: AFP
Putin appears to be one of the most closely-guarded leaders in the world. At Putin 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.
However if what the Kremlin is saying is true, it will raise questions about how well protected the president really is.
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There will also be scrutiny over the effectiveness of Russian air defences. In recent months, anti-aircraft systems have been spotted on Moscow rooftops in the vicinity of key buildings.
They have been placed there because the Kremlin is concerned that Ukraine, or those sympathetic to Ukraine, may attempt to carry out aerial attacks on high-value targets.
Whatever actually happened on Wednesday morning, the question now is how Russia will respond. Some officials have already called for tough action. Russian generals have warned many times of harsh responses to any strikes on Russian territory.
But it is unclear whether Russia has the capacity to carry out meaningful retaliatory strikes, or whether this incident will lead to any significant escalation on the battlefield inside Ukraine.
A Ukrainian presidential adviser told the BBC the incident indicated Russia could be "preparing a large-scale terrorist provocation" in Ukraine.
Mykhailo Podolyak said attacking Moscow made no sense for Ukraine but would help Russia justify its own attacks on civilian targets.
He said any drones flying over locations in Russia were down to "guerrilla activities of local resistance forces".
"Something is happening in RF [Russian Federation], but definitely without Ukraine's drones over the Kremlin," Podolyak said.
US Secretary of State Antony Blinken said he could not validate Russia's accusation that Ukraine had tried to kill Putin, but said he would take anything the Russian presidency said with a "very large shaker of salt".
Mick Mulroy, a former US deputy assistant secretary of defence and CIA officer, told the BBC that if reports of the incident were accurate, it was "unlikely" to be an assassination attempt as Ukraine tracks President Putin's movements closely and he was not in Moscow at the time.
"This may have been to show the Russian people that they can be hit anywhere and that the war they started in Ukraine may eventually come home to Russia, even the capital," he said.
Alternatively, if the reports were not accurate, "Russia may be fabricating this to use as a pretext to target President Zelensky - something they have tried to in the past", Mulroy said.
Russia also noted the alleged drone incident had come shortly before Russia's 9 May Victory Day parade in Moscow, which foreign dignitaries were expected to attend.
The parade will go ahead as planned, Russian officials said.
Moscow's mayor on Wednesday announced a ban on unauthorised drone flights over the city.
Several Russian cities had already announced they would scale back this year's Victory Day celebrations.
Russian authorities have cited security reasons and attacks from pro-Ukrainian forces for the changes. Explosions and fires have occurred in Russia in recent weeks.
