The Belarusian leader called claims that his country's forces would join Putin's as "total nonsense."
Vladimir Putin appeared to drop his pen after a surprising announcement from his Armenian counterpart at a CSTO summit.
Attempts to pressure Minsk to join Putin's invasion are likely to fail, according to the Institute for the Study of War.
Britain's Ministry of Defense said that the deployment of the aircraft and weapon was a "message to the West."
"If the army of Belarus supports the Russian aggression, we will respond," the command of the Armed Forces of Ukraine said on Friday.
Belarusian President Alexander Lukashenko has said there would be a joint Belarusian-Russian force to fight in Ukraine.
Belarusian soldiers should "switch sides" if ordered to join Putin's invasion of Ukraine, Sviatlana Tsikhanouskaya told Newsweek.
Belarus has aided Russia by allowing strikes on Ukraine from its territory, but it has not become directly involved in the conflict by sending troops.
The Belarusian government has begun stockpiling weapons and securing at least 5,000 places of shelter in case of missile attacks.
The political survival of Belarus' president has largely depended on Moscow's support, and Putin exercises"very serious leverage on him," a Russian expert says.
Belarusian President Alexander Lukashenko said that "if nuclear weapons are used even by one country, it will cause a chain reaction."
The lack of reliable and sustainable strike capabilities is one of Russia's biggest issues in Ukraine, British officials said.
The condemnation of Belarus comes against a broader denunciation of Russia and the threat of additional sanctions over its recent missile strikes on Ukraine.
The think tank said it remains "exceedingly unlikely" that there will be Belarusian military involvement in Ukraine on Russia's behalf.
On Monday, the European Commission urged the Belarusian president to stop allowing Moscow to use Belarus as a launch pad for attacks on Ukraine.
Belarusian President Alexander Lukashenko said Friday that his country "will fight only when we have to defend our home, our land."
The U.K. Defense Ministry does not believe training exercises launched Thursday by Belarus indicate that the country is preparing for direct war involvement.
"They [the West] must understand that neither helicopters nor planes will save them if they go for escalation," Lukashenko said,
Ukraine's Defense Ministry tweeted that it wished for the "reunification" of Belarus President Alexander Lukashenko "with his good friends Hussein & Gaddafi."
Lukashenko hailed the "the centuries-old foundation of sincere good-neighborly relations" between the two countries as Ukraine marks its independence.
The independent intelligence group's assessment comes after a reportedly large explosion in Belarus last week destroyed a tank.
Russian strikes fired at Ukraine from Belarus prove Putin's forces "can pass through Belarus quite freely," said Estonian official Marti Magnus.
Ukraine's foreign minister has vowed to sever diplomatic ties with Belarus should its army intervene in the Russia-Ukraine war.
During the Ukraine war Lukashenko has been a key Putin ally, letting Russia fire missiles from Belarus, the UK's defense ministry reported Sunday.
Belarus has yet to send any forces into Ukraine, but it has aided Russia by allowing it to place troops within and launch airstrikes from its territory.
"I ran from Belarus to escape prison, but I don't want to keep running," a Belarusian fighter told Newsweek. "It's better to risk your life as a free person."
The president of Belarus, Alexander Lukashenko, told Agence France-Presse about what he thinks might happen if the war continues.
As a video of Russia's president waiting uncomfortably for his Turkish counterpart goes viral, Newsweek looks back at Putin's own history of tardiness.
The state agency last convened on July 1, when members focused on "the performance of the defense industry."