Boris Johnson has ruled out sending British troops to battle the Russians in Ukraine as the brutal conflict entered its sixth day.
The Prime Minister also repeatedly rejected calls for a no-fly zone over the country despite pleas to stop the brutal bombardment of Ukraine by Russian jets.
Moscow's troops are advancing on Kyiv, with a 40-mile military convoy seen north of the Ukrainian capital. Fears are mounting that the capital will also face heavy shelling already inflicted on several other cities.
But Britain and other NATO countries are refusing to implement a no-fly zone for fear of further escalating tensions with Russia.
The PM came under pressure to do more to help the Ukrainians as he flew out to NATO's eastern flank for crisis talks.
Speaking at a military base in Estonia, Mr Johnson said: "I want to be crystal clear. We will not fight Russian forces in Ukraine.
"Our reinforcements, like these reinforcements here in Tapa, are firmly within the borders of NATO members and that is profoundly the right thing to do."
The PM also rejected calls for a no-fly zone, saying it was "not of the agenda of any NATO country".
He said: "It's very, very important to understand NATO is a defensive alliance. This is a time when miscalculation and misunderstanding is all too possible and it's therefore crucial that we get that message over.
"When it comes to a no-fly zone in the skies above Ukraine we have to accept the reality that that involves shooting down Russian planes ... that's a very, very big step, it's simply not on the agenda of any NATO country."
NATO Secretary-General Jens Stoltenberg warned that "heavy Russian armour" moving towards Kyiv would bring "more death, more suffering and more civilian casualties".
Speaking alongside the PM, he said: "That is the reason why we need to continue to provide support to Ukraine, why we continue to call on Russia to stop this bloody war and why we need to impose costs by the heavy sanctions on Russia."
Mr Stoltenberg added that the situation was "horrifying" and "a blatant violation of international law".
Earlier, Ukrainian journalist Daria Kaleniuk confronted Mr Johnson at a televised press conference in Poland.
Ms Kaleniuk, who had escaped over the border, broke down in tears as she demanded he implement a no-fly zone.
She told the PM: “You’re coming to Poland - you’re not coming to Kyiv, Prime Minister. You’re not coming to Lviv.
“Because you are afraid. Because NATO is not willing to defend.
“Because NATO is afraid of World War 3 - but it has already started."
Mr Johnson thanked her but admitted: “There is not enough we can do as the UK government to help in the way that you want, and I’ve got to be honest about that.”
He later told ITV News there cannot be a no fly zone in Ukraine because “you would have to take decisions to shoot down Russian jets.”
He added: “And that's not something that any Western country is contemplating.
"What we can do is stick up for the Ukrainians, support them in the ways that we are, offer humanitarian relief and assistance, and offer to welcome them."
Labour leader Keir Starmer was urged by Ukrainian parishioners to push the Government to implement a no-fly zone on a visit to the Ukrainian Orthodox Autocephalic Church in London.
Cab driver Andriy Petryk said to him: "I know it's too much to ask, with the no-fly zone. Two weeks ago, most of the country was against switching Russia off from Swift.
"Opinions change on an hourly and daily basis. I think you might reconsider. Would you like to ask (the Government) to implement a no-fly zone?"
Mr Starmer said: "We'll talk to the Government about that."
Speaking to journalist, he said the UK "must supply the equipment and the support that we can" to the Ukrainian people and "double down on sanctions".
"We need not to just isolate Russia, but to make it impossible for Russia to function through the sanctions that we put in," the Labour leader said.
"And of course the humanitarian aid, which is so important and the community here have been reminding me historically how important that aspect is as well."
