LONDON- Britain and other NATO allies must be careful not to give Russia justification to invade Ukraine, Prime Minister Boris Johnson said on Tuesday, adding that no NATO allies were willing to send large numbers of troops to fight in Ukraine itself.

"We have to beware of doing things ... that would constitute a pretext for (Russian President Vladimir) Putin to invade," Johnson said.

"We have to calculate and calibrate what we do very carefully and I think building a strong package of economic sanctions, continuing to supply defensive weaponry, and all the other things that we're doing - that's the right package."

(Reporting by William James, Editing by Kylie MacLellan) ((; +44 207 542 3374; Reuters Messaging: