The British prime minister also said his government has announced the "first phase of a bespoke humanitarian route" for refugees to come to the UK.
British Prime Minister Boris Johnson pledged his support for the people of Ukraine during a trip to Estonia and Poland on Tuesday, but made clear that the UK would "not fight Russian forces in Ukraine".
"I want to be crystal clear finally on that point, we will not fight Russian forces in Ukraine. And our reinforcements, like these reinforcements here in Tapa, are firmly within the borders of NATO members and they are profoundly the right thing to do," Johnson said in Estonia, as he stood alongside Prime Minister Kaja Kallas and NATO Secretary-General Jens Stoltenberg at Tapa Military Base.
Johnson also said his government has announced the "first phase of a bespoke humanitarian route" for refugees to come to the UK.
The British government has been criticised for not matching the European Union, which says it will let Ukrainians stay for up to three years without applying for asylum.
Britain says it will allow Ukrainians in the country to bring their immediate family members to the UK. That applies to partners and children, but not parents or siblings.
Ukrainian journalist Daria Kaleniuk was overcome with emotion as she questioned UK Prime Minister Boris Johnson in a briefing during his preceding visit to Poland.
She urged Britain and NATO allies to enforce a no-fly zone over her country to protect people from Russia's bombardment.
"You are talking about the stoicism of the Ukrainian people but Ukrainian women and children are in deep fear because of bombs and missiles coming from the sky," she said.
"We are asking for a no-fly zone," she added, "NATO is not willing to defend, because NATO is afraid of the World War III. But it is already started."
Johnson said the implication of that would be that the UK forces would be engaged in a direct combat with Russians. "That is not something we can do and that have envisaged," Johnson said.
Johnson said the the UK was the first European country to offer military assistance. He said the government was "doing everything we can to tighten the economic noose around the Putin regime," adding that he thought "it will work".
Boris Johnson thanked Poland for taking in hundreds of thousands of Ukrainians fleeing the violence.
"I am absolutely convinced, I am more convinced than ever, as this hideous conflict progresses, that Putin will fail," he said, adding that he felt "we will succeed in protecting and preserving a sovereign, independent and democratic Ukraine".