David Cameron: Russia may face EU sanctions within days
David Cameron has said Russia could face targeted sanctions "within days" if it does not respond to concerns over its incursion into Crimea.
He announced that EU representatives would meet in London on Tuesday to discuss measures including travel bans and asset freezes against 18 people linked to Vladimir Putin's government.
But Downing Street would not name those who could be affected.
Labour leader Ed Miliband urged Mr Cameron to exert "maximum pressure".
Moscow is continuing to strengthen its grip on Crimea before a secession referendum in Ukraine's southern region on 16 March.
The UK prime minister said Moscow had to be shown that it could "not trample over" Ukraine. He called the referendum "illegal and illegitimate" and "farcical", as there was no time for campaigning, and movement around Crimea was restricted by the presence of military.
Addressing the House of Commons, Mr Cameron said: "Britain's own security and prosperity would be at risk if we allow a situation where countries can just flout international rules without incurring consequences."
The prime minister has spoken to US President Barack Obama about the crisis and he held meetings with German Chancellor Angela Merkel on Sunday night.
He said: "No one should be interested in a tug of war. Ukraine should be able to choose its own future and act as a bridge between Russia and Europe.
"In Europe we have spent the last 70 years trying to keep the peace and we know from history that turning a blind eye when nations are trampled over stores up trouble for the longer term.
"We must stand up to aggression, uphold international law, and support the Ukrainian government and the Ukrainian people who want the freedom to choose their own future."
Last week, the European Union decided to take measures against Russia. They include suspending negotiations with Russia on establishing visa-free travel in Europe and suspending preparations for this summer's G8 meeting of leading economies in Sochi, Russia.
The EU also said it would consider asset freezes and travel bans if there was not further diplomatic progress, and trade and financial sanctions if Russia made further incursions.
EU representatives are to hold a meeting in London on Tuesday to draw up a list of 18 people "linked to" the Russian government who would be the first to face travel bans and asset freezes.
Downing Street refused to speculate about whether this would include politicians and diplomats or business leaders, but it is not thought to include Mr Putin.
Mr Cameron, who was briefing MPs about last week's EU summit, said: "If further steps are taken to destabilise Crimea, there will be far-reaching consequences."
Mr Miliband told MPs: "I would urge him, particularly as we approach the referendum in Crimea, to apply maximum influence on our allies so that maximum pressure can be applied on the Russian government.
"Hesitancy or weakness on the part of the EU about its response will send precisely the wrong message and the UK has a vital position of responsibility in making sure that does not happen and instead that the EU and the US stand together in clear and united resolve. "
On Sunday, tens of thousands of people across Ukraine held rival pro-unity and pro-Russian rallies.