Britain reserves the right to arm Ukraine and will not allow the Ukrainian army to collapse, Foreign Secretary Philip Hammond told parliament on Tuesday.
"It's a national decision for each country in the NATO alliance to decide whether to supply lethal aid to Ukraine," he said in a statement to MPs.
"The UK is not planning to do so but we reserve the right to keep this position under review," he said.
Hammond said Russia had "aggravated the effects of its initial incursion" by providing military support to "proxies" fighting in war-torn eastern Ukraine.
Britain and its NATO allies "share a clear understanding that, while there is no military solution to this conflict, we could not allow the Ukrainian armed forces to collapse," he said.
The statement follows US President Barack Obama's announcement on Monday that he would hold off making a controversial decision on sending arms to Ukraine until efforts led by France and Germany to broker a ceasefire with Russia were given a chance.
The leaders of Ukraine, Russia, France and Germany meet in Minsk on Wednesday in a bid to hammer out a peace deal in a 10-month war that has killed over 5,400 people.
Hammond said Russia had sent hundreds of heavy weapons and hundreds of soldiers, including special forces, into eastern Ukraine.
"The recent escalation in fighting would not have been possible without the military support and strategic direction that Russia provides," he said.
Kiev also accuses Moscow of supplying and training the heavily armed separatists, but Russia denies this.
A rocket attack on Ukraine's military headquarters in the town of Kramatorsk in the country's east killed at least six civilians Tuesday, officials said.