Vladimir Putin is being 'driven nuts' by Ukrainian president Volodymyr Zelenskyy’s heroism, a former US ambassador has said.

Russian invading forces have been met with fierce resistance in Ukraine, whose strong line of defence has pushed the onslaught into its seventh day.

Zelenskyy heroism in the face of seemingly overwhelming odds has been widely praised after he remained in Kyiv to rally his people against the Russian incursion.

Former US ambassador to Russia Michael McFaul said his sources in the region had told him Putin was not happy with the reaction his counterpart was receiving.

“The fact that Zelenskyy is not leaving Kyiv is driving Putin nuts," he told NBC’s Morning Joe.

“Because the whole world is talking about his heroism and the whole world is talking about how evil Vladimir Putin is.”

Zelenskyy has released a number of videos proving he was still on the ground in Kyiv after rumours he had fled.

“We are here. We are in Kyiv. We are defending Ukraine,” he said in a clip released last Friday.

He also later rejected an offer from the US to evacuate him, adding: “I need ammunition, not a ride.”

The Times reported a bounty on his head, saying more than 400 Russian mercenaries were trying to assassinate him.

Speaking on Wednesday, the seventh day of the invasion, the president said Russia was aiming to erase Ukraine, its history and people.

Unshaven and wearing a khaki T-shirt, said the West's response was not enough, calling for more international support, including backing Ukraine's bid to join the European Union.

"This is no time to be neutral," he said.

McFaul, who served as the United States Ambassador to Russia from 2012 to 2014, said that the Russian president had finally "overreached" after two decade in power that had previously seen him successful in multiple conflicts. Most notably, Moscow annexed Crimea from Ukraine in 2014 and the regions of South Ossetia and Abkhazia from Georgia in 2008.

"When dictators hold on too long, they get disconnected from reality, they don’t listen to their advisers. That’s exactly what’s going on here," McFaul said.

The former ambassador also claimed he had recently spoken to a contact in Russia, one who has known Putin and the Moscow elite for 30 years, saying those around the president were “appalled” at how disconnected Putin seemed from reality.

Putin, McFaul said, has in reality been isolated for many years, doesn't listen to anyone, rarely goes to work and often stays on his compound.

But the Ukraine war has changed the landscape in Moscow, with wealthy oligarchs and senior political figures now subjected to swingeing sanctions who McFaul says are "in shock" at the fallout from Putin's decision to invade a neighbouring country and spark the wrath of Nato and Europe.

But Putin's isolationism is also a cause of alarm among Western officials, with US intelligence agencies worried Putin is so frustrated by setbacks in Ukraine that he could significantly increase the violence.

The Russians have so-far targeted built-up areas in Kyiv, Kharkiv, Mariupol and Chernihiv, but have been largely held by Ukrainian armed forces as well as citizens, many of whom have taken up arms.

In Kyiv, Tuesday evening's missile strike on a TV Tower killed five people while a 40 mile-long convoy of soldiers is on its way towards the capital, though US officials have claimed it has made little progress in the past 24 hours, frozen in place by logistical and supply problems.

However, UK defence secretary Ben Wallace warned Russia will now intensify its campaign, with indiscriminate carpet bombing tactics and a scale of brutality that "is going to get worse".