Russian President Vladimir Putin warned Ukraine on Monday that there would be a harsh response if there were any further attacks such as that on the Crimean Bridge.

"If attempts to carry out terrorist attacks on our territory continue, Russia's responses will be harsh and, in terms of their scale, will correspond to the level of threats posed to the Russian Federation. No one should have any doubts about this," Putin said at the opening of a meeting of Russia's Security Council.

"Forensic and other data, as well as operational information, indicate that the explosion on Oct 8 was a terrorist act — a terrorist attack aimed at destroying Russia's critical civilian infrastructure. It is obvious that the Ukrainian secret services ordered, organized and carried out the terrorist attack aimed at destroying Russia's critical civilian infrastructure."

The Russian president accused the Ukrainian government of putting "itself on a par with international terrorist groups".

Loud explosions rocked the Ukrainian capital Kyiv as air raid sirens sounded across the country on Monday morning, a day after the Russian leader denounced the Crimean Bridge explosion as terrorism.

Kyiv Mayor Vitali Klitschko said the capital came under attack after at least five explosions were heard near the city center just after 8 am.

Subway services have been suspended, according to an official alert from the city's metro system. Stations across Kyiv have been crowded with residents taking shelter since the first strikes began.

Ukrainian public broadcaster Suspilne quoted emergency services as saying there were dead and wounded in Kyiv, but gave no further details of casualties. Explosions were also reported in Lviv, Ternopil and Zhytomyr in Ukraine's west, and in Dnipro in central Ukraine.

The Ukrainian Ministry of Defense vowed on its Twitter account to issue a strong response.

In Russia, Reuters news agency reported that a loud explosion was heard on Monday morning in the Belgorod region, near the border with Ukraine.

Of the bridge attack and a vehicle seen in video footage, Russian Investigative Committee chairman Alexander Bastrykin said: "We have already established the route of the truck which exploded. It drove through Bulgaria, Georgia, Armenia, North Ossetia and the Krasnodar Territory.

"We have also identified the carriers.... With the help of FSB (Federal Security Service) agents, we were able to identify suspects from among those who could have arranged the terrorist act and those who are active within the Russian Federation."

The bomb attack at the Crimean Bridge was a special operation carried out by the Security Service of Ukraine, the Interfax-Ukraine news agency cited a source in Ukraine's law enforcement agencies as saying.

The SSU hasn't commented on the incident. But the Ukrainian postal service has already announced a series of stamps celebrating the destruction of the bridge over the weekend.

Images of the stamps were released just hours after the attack on the bridge along with a giant replica stamp featuring an artist's rendition of the attack, the Russian state news agency TASS reported.

Kremlin spokesman Dmitry Peskov did not assert on Sunday the possibility of Russia using nuclear weapons in retaliation for the bridge explosion, Russia's RIA Novosti news agency reported.

Still, Putin held an operational meeting of his Security Council on Monday.

The meeting itself isn't out of the ordinary, as Putin regularly holds operational meetings with the Security Council, usually on a weekly basis, according to TASS.

US President Joe Biden spoke with German Chancellor Olaf Scholz by phone on Sunday, discussing sanctions on Russia as well as assistance to Ukraine.

In a statement from the White House, Biden and Scholz expressed their "commitment to hold Russia accountable for its brutal actions and to provide security and economic assistance to Ukraine".

Chinese Foreign Ministry spokeswoman Mao Ning said on Monday, after the explosions in Kyiv, that it is important for all parties to stick to dialogue and negotiations and work together to de-escalate tensions.

China has maintained communications with all parties, Mao said, adding that the country hopes to play a constructive role in promoting de-escalation.

Agencies contributed to this story.