The Russian military on Wednesday escalated its use of air and missile strikes against major cities where local military and civilians have mounted a more formidable defence than the Kremlin may have anticipated.

Poroshenko, armed with a handgun and wearing military clothing during his interview, said the enforcement of a no-fly zone "prohibited and protected by NATO" in Ukrainian airspace could shift the course of the war.

That request has not been entertained by various NATO officials because enforcement of a no-fly zone could amount to a direct confrontation between NATO and the Russian military.

"Putting in place a no-fly zone would be a severe escalation on the part of NATO and it is not on the table at the current time," Defence Minister Anita Anand said Tuesday.

Russian President Vladimir Putin has in recent days pointed to his nation's nuclear arsenal in a bid to deter direct Western intervention in Ukraine.

No-fly zone might have given Putin 'second thoughts'

Ukrainian soldiers have successfully shot down some Russian planes and helicopters during the first week of the conflict. The Ukrainian military accomplished those victories after being armed by NATO members with various anti-aircraft and anti-tank weapons.

"If we can do it, why we cannot do it together with NATO?" Poroshenko asked.

The former president also pointed to NATO's enforcement of a no-fly zone above Libya in 2011 during that country's civil war. He asked why a similar measure is not being considered now.

Retired general Rick Hillier, former commander of the Canadian military, has described a no-fly zone as a necessary response to Russian aggression.

"I know that NATO is a defensive organization but you don't start defence at your front door," Hillier said on Tuesday.

"That no-fly zone … Putin might have had second thoughts before he launched if we had done it."

Strategy against Russia not yet clear, says former commander

In addition to the risk of escalating a conflict with a nuclear superpower, others point to logistical and strategic challenges involved in establishing a no-fly zone.

Steve Day, former commander of Canada's Joint Task Force 2, said that any single member of NATO could veto a decision to implement a no-fly zone outside NATO territory.

He also said the United States has the only air force capable of successfully enforcing a no-fly zone, meaning any such decision would have to come from Washington.

Day said a no-fly zone should not even be considered until NATO members agree on a unified response to the crisis.

"I've yet to hear a strategy about how we're going to support and defend Ukraine," Day told CBC's Power & Politics.

"You've got to be very serious about whether you're going to engage those Russian aircraft or not."

Despite a strong push by Russian forces towards Kyiv in recent days, Day predicted that Moscow's goal of toppling the government will be hard to achieve.

Poroshenko indicated that Ukrainians will keep fighting until the Russian forces withdraw.

"I can tell you that every single town, every single street, every single quarter and every single house would be a Ukrainian fortress and would be hell for the Russian soldiers," he said.