Confidence "cannot be high" that the latest ceasefire in Ukraine will succeed, Dr Liam Fox has warned as he said further pressure on Russia was needed if it does not hold.
The former Conservative defence secretary described the situation as a "crisis made in Moscow", adding that "Russia is very clearly now an aggressive power".
An agreement brokered by German chancellor Angela Merkel and French president Francois Holland and struck at Minsk in Belarus is set to see a ceasefire from today.
Speaking on Sky News' Murnaghan programme, Dr Fox said: "We wait to see, but I'm afraid that the word of President Putin is not worth very much, it hasn't been in the past. Everyone hopes that this is going to be a breakthrough but I am afraid that confidence cannot be high."
He added: "If it doesn't hold there needs to be further pressure on Russia. Remember, this is a crisis made in Moscow, Moscow annexed the Crimea, we saw the shooting down of the Malaysian airliner with a Russian missile, a Buk missile, which we know that Russia has been giving to the armed rebels in Ukraine. It's Russia that needs to step back and leave the Ukraine in peace."
Asked if Britain should send lethal weapons to Ukraine if the ceasefire fails to remain in place, Dr Fox said: "Well, we'll be faced with a very difficult choice because if we were to do nothing and Russia were to continue its aggression, then you could see the dismemberment of Ukraine, that would have a major impact on wider security.
"We know that Russia is giving three particular types of weapons to the Ukrainian rebels, that is secure communications, new heavily armoured vehicles and also anti-aircraft missiles.
"I t would be, I think, prudent to ensure that Ukraine was able to counter all three of these things for its own security. Remember, Ukraine is the country that has been attacked here, it is not the aggressor."
Dr Fox argued it was "very clear" that Russian president Vladimir Putin "wants to be able to create client states which potentially will destabilise European security", adding: "I think we've been very lax for a long time."
He went on: "I think it's time for us to recognise that Nato in particular needs to show greater solidarity, we will need to increase our military exercises, we will need to increase our number of bases in some of our Nato allies and of course we'll all have to ensure that we spend what we are committed to in terms of wider Nato defence."
Dr Fox said one of the West's weaknesses was the fact that "not all our allies are willing to play their full role", adding that appeasement had a "bad track record".
He said: "Some countries seem to believe that they should get their insurance policy without ever paying the premiums, that's something that we have to deal with at a political level, so political solidarity as well as showing that we are willing to use the force that we have if necessary, it's what is required to stop this."
Britain, he argued, had been at the "forefront" of putting the economic pressure on Russia and would be playing a major role to "get the United States and our Nato allies in line".
David Cameron earlier urged European Union leaders to stand firm on the need to maintain sanctions against Russia as signs of division emerged following the Ukraine peace deal.
The Prime Minister said it was "actions on the ground rather than just words on a piece of paper" that mattered, with the UK supporting a tough stance on the sanctions regime.
British officials said the apparent diplomatic breakthrough would be treated with caution until there was proof that President Putin had changed course.
Mr Cameron, who was in Brussels for a European Union summit, is understood to have been one of the leading opponents of a suggestion at the summit to weaken the sanctions regime against Russia by postponing the imposition of measures against 19 people due to take effect tomorrow.
A Downing Street spokesman said: "The Prime Minister called President Poroshenko this afternoon to discuss the situation in eastern Ukraine.
"Both leaders expressed concern about the situation on the ground. While the level of fighting has lessened considerably since the ceasefire came into force at midnight, there have been multiple violations in particular around the town of Debaltseve. They agreed on the importance of both sides respecting the ceasefire.
"They discussed what role the United Nations Security Council could play in supporting the ceasefire and the agreements reached in Minsk in September 2014 and February 2015, and agreed on the importance of underlining Ukraine's territorial integrity. They agreed to stay in close touch in the coming days."