Economic sanctions against Russia should be extended and toughened if Russian President Vladimir Putin does not pull back from Ukraine, the Prime Minister has said.
Speaking at a liaison committee meeting, David Cameron said if "miraculously" Russia withdrew its weapons and upheld ceasefires, the sanctions would be lightened - but in the meantime, he said the UK, US and Europe had to take a stand against his actions.
Mr Cameron said the sanctions were necessary to protect Moldova and the Baltics from being destabilised next.
He also criticised "serious" violations of British airspace by Russian craft, but said the UK was "more than capable" of defending itself.
I'm sure the Russian's would like us to react in a more volumous way about it.
So far this year the RAF has scrambled the quick reaction aircraft twice, last year it was eight times, if you go back to 2011 it was 10 times, and in 2007 it was 19 occasions.
I think we should be strong, measured and clear, but we should be absolutely confident in our air force, in our Typhoons and the pilots that we have. We are more than capable of protecting our airspace.
Thousands of Russians are marching in Moscow to mark the first anniversary of what they viewed as a "coup" in neighbouring Ukraine.
14 soldiers died and many more were injured as Ukraine left the strategically important hub of Debaltseve.
Dramatic footage shows the moment a TV crew were forced to flee a huge explosion after a gas pipeline was hit by mortar fire in Ukraine.