Ukraine crisis: Russian actions 'cannot stand' says PM
No options should be ruled out in efforts to exert pressure on Russia over its incursion into Ukraine, Prime Minister David Cameron has told MPs.
Russia's "dismembering" of the Crimean peninsula was unacceptable and "could not be allowed to stand", he said.
He urged EU leaders to send a "clear and united" message about the consequences of Moscow's actions when they meet on Thursday.
Labour leader Ed Miliband said economic sanctions should not be ruled out.
This came as Foreign Secretary William Hague, US Secretary of State John Kerry and the Ukrainian foreign minister agreed that international observers should be deployed immediately in Ukraine following a meeting in Paris.
At Prime Minister's Questions, Mr Miliband condemned Russia's actions and promised the government his party's support to tackle what he said was the biggest crisis facing Europe since the war in Kosovo.
He said the UK should consider responding by targeting wealthy Russians, doing business in London and "shopping in Selfridges", by examining asset freezes and visa restrictions.
A document, photographed on Monday as a senior official carried it into a meeting in Downing Street, warned against sanctions, citing the extent of Russian commercial interests in London.
The BBC's political editor Nick Robinson said it was "very hard" to see the EU agreeing trade sanctions but that targeted action against Russians with links to the Kremlin could be announced in the coming days.
Mr Cameron, who will speak to US President Obama later, said the UK should not discount any action in partnership with its allies, suggesting that as it stood now he could not see June's G8 summit in Sochi taking place.
It was, he insisted, in the UK's national interest to see the rule of law upheld around the world and, in that respect, Russia taking effective control over part of Ukraine's sovereign territory was "unacceptable".
"There are further steps we should consider, even as we look at the current status quo," he said.
"But we also need to consider what extra steps, what extra political, economic and diplomatic steps can be taken to discourage Russia from taking further steps in terms of dismembering and disrespecting the territorial integrity of Ukraine."
Bringing Russian and Ukrainian officials together for talks was the most effective way of defusing tensions, he added.
"There has been some progress in putting together a contact group to start having a group of countries around Russia and Ukraine to encourage such dialogue to take place."
The Labour leader said "if we do not see the required action from Russia, we should look at asset freezes and travel restrictions on designated individuals so that Russia is clear about the consequences of its actions".
Meanwhile, senior Conservative and Lib Dem politicians said it was "inexcusable" for details of the UK's strategy to have become public in the way they did and that it had "weakened" the UK's position.
"In circumstances like this where there is detailed and difficult negotiation, the last thing you want to do is to show your hand," said former Lib Dem leader Sir Menzies Campbell. "Ambiguity is absolutely fundamental.
"The prime minister sought to repair all that by saying 'well, everything is on the table' but it will be quite difficult to go back. That is the weakness that has been presented."
Russian President Vladimir Putin has said he is protecting the Russian-speaking majority in Crimea, which was part of Russia until 1954, describing the military mobilisation as a "humanitarian mission" in response to the overthrow of Ukraine's president Viktor Yanukovych last month.