Belarusian President Alexander Lukashenko says he has ordered troops to deploy jointly with Russian forces near Ukraine, in a further dramatic escalation of the conflict.

The comments from Mr Lukashenko, who has held power in Belarus since 1994 and is Vladimir Putin’s closest ally, followed Russia’s deadly missile strikes on Ukraine on Monday.

Russia used Belarus as a staging ground early in the war but so far Mr Lukashenko has not sent in his troops. His latest comments – as he accused Kyiv of planning attacks on his nation – signalled a possible combined Russian-Belarus joint force in Ukraine’s north.

“Strikes on the territory of Belarus are not just being discussed in Ukraine today, but are also being planned,” Mr Lukashenko said at a meeting on security on Monday (local time).

“Their owners are pushing them to start a war against Belarus to drag us there.”

He provided no evidence for his claims, but said he had spoken to the Russian president about the issue at a recent meeting in St Petersburg.

“We have been preparing for this for decades. If necessary, we will respond,” Mr Lukashenko said.

He said Belarus had received a warning through unofficial channels that Ukraine planned “Crimean Bridge 2”, though he did not give details.

“My answer was simple: ‘Tell the president of Ukraine and the other lunatics: If they touch one metre of our territory then the Crimean Bridge will seem to them like a walk in the park’,” Mr Lukashenko said.

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At the weekend, the bridge linking Russia to the annexed Crimean peninsula – a key project of Mr Putin – was damaged. Ukraine, which views the bridge as a military target sustaining Russia’s war effort, has celebrated the blast without claiming responsibility.

Poland was one of the first nations to react to the new threat, urging its citizens in Belarus to leave the country.

“We recommend that Polish citizens staying on the territory of the Republic of Belarus leave its territory with available commercial and private means,” the Polish government said in guidance for travellers published on its website.

Relations between Warsaw and Minsk worsened last year, and have deteriorated further this year since Russia’s invasion of Ukraine.

On Tuesday (Australian time), Ukraine vowed to strengthen its armed forces after Russia launched its biggest aerial assaults on cities since the beginning of the war, forcing thousands to flee to bomb shelters and prompting Kyiv to halt electricity exports to Europe.

Missiles hit cities across Ukraine on Monday morning, killing 14 people and injuring scores, as they tore into intersections, parks and tourist sites.

Explosions were reported in Kyiv, Lviv, Ternopil and Zhytomyr in western Ukraine, Dnipro and Kremenchuk in the centre, Zaporizhzhia in the south and Kharkiv in the east, Ukrainian officials said.

The barrage of dozens of cruise missiles fired from air, land and sea was the most widespread wave of air strikes to hit away from the front line, at least since the initial volleys on the war’s first day, February 24.

Mr Putin ordered “massive” long range strikes after the Crimean bridge attack, and has threatened more strikes if Ukraine hits Russian territory.

Russia has called the deadly explosion “terrorism”.

“To leave such acts without a response is simply impossible,” Mr Putin said, alleging other, unspecified attacks on Russian energy infrastructure.

Ukraine’s Volodymyr Zelensky spoke to US President Joe Biden on Monday and wrote on Telegram afterwards that air defence was the “No.1 priority in our defence co-operation”.

“We will do everything to strengthen our armed forces,” he said in a Monday night address. “We will make the battlefield more painful for the enemy.”

Mr Biden told Zelensky the US would provide advanced air defence systems. The Pentagon said on September 27 it would start delivering the National Advanced Surface-to-Air Missile System during the next two months or so.

Moscow’s attacks caused thousands of Ukrainians to race to bomb shelters as air raid sirens rang out.

Mr Zelensky said 12 people were killed and officials have reported scores of injured after the assaults.

The attacks were deliberately timed to kill people and knock out Ukraine’s power grid, he said. His prime minister reported 11 major infrastructure targets were hit in eight regions, leaving parts of the country with no electricity, water or heat.

As it tried to end blackouts, Ukraine halted electricity exports to the European Union, at a time when the continent already faces surging power prices that have stoked inflation, hampered industrial activity and caused sky-high consumer bills.

With troops suffering weeks of setbacks on the battlefield, Russian authorities have faced the first sustained public criticism at home of the war, with commentators on state television demanding ever tougher measures.

Since early September, Ukrainian forces have burst through front lines and recaptured territory.

Mr Putin responded by ordering a mobilisation of hundreds of thousands of reservists, proclaiming the annexation of occupied territory and threatening repeatedly to use nuclear weapons.

On Saturday, Russia’s defence ministry named General Sergei Surovikin, who won acclaim in Syria, as commander of Russian forces in Ukraine. A Russian air campaign in Syria helped the government crush its enemies.

-with AAP