David Cameron promised to "stand up very strongly" for the Ukrainian people as he met a delegation of the country's politicians including former heavyweight boxer Vitali Klitschko.
The Prime Minister welcomed the delegation to Downing Street and told them the UK would continue to speak out about the "unacceptability" of Russian behaviour.
After the meeting, Mr Klitschko said "we expect support from Great Britain" and draped himself in the Ukrainian flag as he spoke to demonstrators demanding action against Russia.
Mr Cameron said: "Britain will stand up very strongly for the Ukrainian people to determine their own future, a future that we hope you will choose to have a close relationship with countries like Britain and the European Union but a future in which you also choose to have a strong and positive relationship with Russia."
He added: "We will continue to send very tough messages about the unacceptability of Russian behaviour."
In the Commons, Mr Cameron warned f ailure to stand up to President Vladimir Putin over the Crimea would lead to further "unacceptable behaviour" by Russia in other neighbouring countries.
He reaffirmed Britain's commitment to the guarantees given to the Baltic states and to Poland as members of Nato - which regards an attack on one alliance member as an attack upon all.
Following talks of the G7 group of leading industrial powers this week in The Hague and last week's EU summit in Brussels, he said that the international community stood ready to to intensify sanctions against Moscow if there was any escalation of the situation.
"Russia's violation of international law is a challenge to the rule of law around the world and should be a concern for all nations," Mr Cameron told MPs.
"We have to be clear how unacceptable it is and to see through these economic sanctions and consequences, otherwise we will face similar situations in similar countries with a similar sort of unacceptable behaviour.
"Britain must continue to play its part in standing up to Russia's actions, pressing for Russia to change course, and helping the Ukrainian people in their hour of need."
Amid concerns that Mr Putin could mount similar incursions against the Baltic states of Latvia, Lithuania and Estonia, Mr Cameron confirmed that the Government was sending four RAF warplanes to the region to assist in "air policing".
"We should do everything we can to reassure our friends and colleagues in Latvia, Lithuania and Estonia and in Poland that we really believe in their Nato membership and the guarantees that we have given them," he said.
Following informal talks between the Russian and Ukrainian foreign ministers in the margins of the nuclear security summit in The Hague, Mr Cameron said a diplomatic resolution was still possible to a crisis triggered by Russia's effective annexation of Crimea following the overthrow of the pro-Moscow government in Kiev.
But he stressed that the international community would move to wide-ranging economic sanctions - on top of the asset freezes and travel bans already imposed by the EU and the US - if there were any further incursions.
"The international community remains ready to intensify sanctions if Russia continues to escalate this situation," he said.
"Russia has a clear choice to make.
"It does not have to continue on this path.
"Diplomatic avenues remain open and we encourage the Russian government to take them."
Foreign Secretary William Hague welcomed the announcement that the Organisation for Security and Co-operation in Europe (OSCE) will send a 100-strong special monitoring mission to Ukraine, which he said "could help make a significant contribution to de-escalate the situation".
Mr Klitschko said the talks in Downing Street focused on efforts to stabilise Ukraine and the country's future relationship with the "family of European countries".
They also discussed the situation in eastern Ukraine, following reports Russia had been massing troops near the border, and Crimea.
Asked if he had pushed Mr Cameron for stronger sanctions, the former boxing world champion said: "We expect support from Great Britain."
He said he expected support because the UK had signed the 1994 Budapest Agreement, along with Russia and the US, which guaranteed the country's integrity in return for it giving Moscow its nuclear stockpile.
Independent MP Petro Poroshenko, seen as a potential presidential rival to Mr Klitschko, said: "We were satisfied by the very strong statement today by Prime Minister Cameron in the House of Commons."
He said further sanctions against Russia could "help us stop the aggression".
The European Commission has been asked to draw up plans for a third stage of measures against Russia, and Mr Poroshenko said: "We understand and we have received information the third stage of sanctions is on the table."