Russia faces further diplomatic and political isolation if it intervenes in eastern Ukraine, David Cameron has warned.
The Prime Minister was meeting world leaders in an effort to ensure Russian President Vladimir Putin was sent a message about his actions.
Speaking in The Hague, where he was meeting Chinese President Xi Jinping and UN Secretary-General Ban Ki-moon, Mr Cameron said he wanted to make sure the international community spoke "with vigour" about Moscow's behaviour.
The PM and other G7 leaders effectively froze Russia out of the elite group at crisis talks last night following the annexation of Crimea by Moscow.
Concerns have been raised, including by Nato's supreme commander US General Philip Breedlove about Russian troops massing on Ukraine's border amid fears that Moscow may make a grab for the Russian-leaning east of the country.
Mr Cameron said: "What I think we need to do is just send the clearest possible message about the eastern Ukraine and the steps that we don't want to see Putin take.
"We need to send a very clear message that would lead to significant sanctions.
"What we have seen at this G7 meeting last night is just a growing sense that if Russia continues to behave like this it will face growing political and diplomatic isolation."
He said the meetings with Mr Ban and Mr Xi were part of an effort to "make sure everyone is communicating this message with vigour".
Asked whether there was any hope that Russia would withdraw from Crimea, he said: "I think there was a very clear consensus that what had happened was wrong and that there needed to be consequences from it, and we couldn't just accept the status quo - the new status quo.
"That is highlighted by the fact that the European Union has said that goods from the occupied Crimea will attract special tariffs.
"There's a view that the status quo is unacceptable, but there's then another very, very strong view that any further steps into eastern Ukraine would be even more serious and would result in much greater sanctions."
Foreign Secretary William Hague has spoken by telephone to interim Ukrainian prime minister Arseniy Yatsenyuk to outline UK support.
A Foreign Office spokesman said: "The Foreign Secretary spoke with Ukraine's Acting Prime Minister Yatsenyuk this morning to assure him of the UK's support for Ukraine ahead of the United Nations General Assembly Resolution on Thursday.
"He also reiterated the G7's commitment that the G8 in Sochi will not go ahead without Russia demonstrating that it is taking steps to de-escalate the situation and to further G7 work on long-term energy policy.
"The Foreign Secretary also took the opportunity to commend acting Prime Minister Yatsenyuk on the restraint shown by the Ukrainian defence forces in the face of great provocation.
"The Foreign Secretary welcomed the meeting between Russian and Ukrainian Foreign Ministers in The Hague yesterday. He said the UK continues to urge the Russian Government to enter into direct talks with the Ukrainian Government, in order to resolve this dispute peacefully.
"He also made clear that the UK would send a clear, strong, consistent message to Russia, and that European nations had made clear that they would respond with another round of wide-ranging measures if there is any further destabilisation in Ukraine by Russia."