Retired Major John Spencer, chair of urban warfare studies at West Point's Modern War Institute, urged civilians to "keep resisting" as the battle to control Kyiv intensifies.

He told Sky News "the challenges are only going to increase" now that Ukrainians have "successfully hurt the Russian army".

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And that while "very small light forces" were only seen initially in Kyiv, Ukrainians must prepare for escalating violence.

But he added that Ukrainians are at an advantage because "defence is the strong form of war".

"It takes five times the amount of force - five soldiers to one person inside the urban terrain - to deal with an urban defender," he said. "They have all the power."

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Razor wire, roadblocks, ambush: 'Turn Kyiv into a fortress'

Speaking from experience, he recommended that civilians "turn Kyiv into a fortress" by building thousands of obstacles to block roads, as well as find protected positions from which to attack Russian forces.

He said bridges can be destroyed, streets can be filled with things like cars, trucks, concrete, and trash - and razor wire could be strung between buildings to damage tracks and wheels of invading forces.

"The goal has to be first to slow them down, stop them - and then if you're armed, to cause injuries, destroy vehicles, kill soldiers," he said. "One of the purposes of a roadblock is to get a vehicle to stop and somebody has to get out of their vehicle."

But for streets Ukrainians still need to use, his tactics would be to build an S-patterned obstacle that still slows vehicles down.

He likened it to making a police checkpoint - and said he has already seen an example of this being done by people on the ground in Ukraine.

Finding protected positions and making 'mouse holes'

Major Spencer said any civilian taking on forces that have stopped or slowed at obstacles must avoid being exposed. Rather than standing out in the open, stay at windows, behind cars, and around alleyway corners.

"I'm still seeing videos of small groups of civilians carrying AK-47s, just walking around in the open," he said. "That's going to become a very easy target."

He advised staying in concrete buildings as opposed to those made of glass or materials that can easily catch fire.

"Take a sledgehammer and do your best to put a small hole in a wall and then aim your weapon without sticking the barrel outside of that hole," he said.

He recommended targeting the vulnerable points of armoured vehicles with Molotov cocktails - ideally from a window above - and focusing on the windows and doors of other vehicles using guns.

Hobby drones could be used to scare Russian troops, he said. "You are not a sniper, but you can put fear in their hearts if they think there are snipers everywhere."

Stay in a group, keep busy, and prevent despair

He said people should form groups of ideally three to five people - with two groups working together from concealed and protected positions.

"Three people do a lot and one of the reasons I want them to group together is a lot of warfare is mental. These people are scared. And they should be.

"We're all scared in combat. One of the things that we know across the history of warfare that helps is having somebody by your side."

He said staying busy keeps "despair" from seeping in, and suggested more activities like moving supplies around and helping others.

He said it is important to stay hydrated, adding: "Three days without water and you won't be able to fight."

Going out safely

He said Ukrainians should find ways of identifying each other so they can go out safely without getting attacked by their own people - things like putting yellow banners around their arms.

If going to get supplies or check on people, it is safer not to take a weapon.

"If you carry a weapon, you're a part of the war," he said. "If you're not, you're technically protected under the law of armed conflict. And right now, Russia is abiding by that."

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'Keep hope'

He urged Ukrainians "fight smart", saying that if guerrilla tactics are done right, they can be used to protect Kyiv and its government building - the most important objectives.

"They have a real possibility to win and cause Russia to withdraw," he said. "Keep hope. Keep fighting."