David Cameron has renewed his call on Moscow to respect Ukraine's territorial integrity in a telephone call with Vladimir Putin.
The Kremlin, which has granted shelter to Ukraine's toppled president Viktor Yanukovych, has come under fire over provocative military drills near the border.
Ukraine has also accused Moscow of orchestrating "an armed invasion" after two airports were taken over by unidentified armed men.
No 10 said the premiers agreed that free and fair elections were the best way to secure Ukraine's future.
A Downing Street spokeswoman said: " The Prime Minister called President Putin today to discuss the situation in Ukraine, particularly the escalating tensions in Crimea.
"The Prime Minister emphasised that all countries should respect the territorial integrity and sovereignty of Ukraine. President Putin agreed, stressing that Russian military exercises in the area had been planned before the current situation in Ukraine.
"The two leaders discussed how the international community could support Ukraine on the path to stability. They agreed that the free and fair elections that the interim government has pledged to hold are the best way to secure a positive future for Ukraine in which all Ukrainian people are represented. A future that would not involve forcing the Ukrainian people to make a choice between Europe and Russia.
"They agreed the international community should also consider how to help the interim government tackle the economic situation.
"They plan to keep in touch on the issue."
The Foreign Office has issued fresh travel guidance advising against all visits to the Crimean Peninsula and calling on those already there to leave.
It warns against using Simferopol airport, which is reportedly operating as usual despite being seized by armed men earlier today, but train routes are still operating.
Britons who choose to remain should keep a low profile, avoid areas of protest or stand-off and stay indoors where possible, the advice states.
The British Embassy in Kiev is now open to the public by appointment only.
A defiant Mr Yanukovych made his first public appearance since fleeing Kiev on Saturday and vowed to "kee p fighting for the future of Ukraine".
At a press conference in the Russian city Rostov-on-Don he said he was "forced" to leave the country after his family received threats but U kraine's prosecutor general has said it is preparing to seek his extradition.
Meanwhile, former US presidential candidate Senator John McCain said Mr Putin sees the United States as weak after the failure to act over Syria.
He told Channel Four News: "Putin views Crimea especially as part of Russia....he views the US as weak, he sees our failures in Syria, Iraq and I think he is emboldened by that and he is going to take steps to try to ensure that Ukraine, particularly Crimea, will remain part of Russia.
"There will be violence in Crimea and Eastern Ukraine...there need to be actions taken - including bringing Georgia into Nato and Moldova into the EU."
He added: "We are working on a billion dollar package - we hope to get that done next week....we need to rescue the Ukrainian economy which is literally on the brink of collapse....maybe ask some of these oligarchs to give back some of the billions they've made with this kleptocracy and corrupt government that they've made so much money off of.
"You have to understand Vladimir Putin believes Ukraine and Crimea are part of Russia and he is not going to go quietly into the night.
"The US President's statement the other day that this is not a Cold War chess board - well to Putin it is.
"This is not a return to the Cold War, we are not going to be on the brink of a nuclear exchange but what I would do - the President can speak out, we can galvanise the international community, we can pass sanctions, we can impose other economic penalties. "
Foreign Secretary William Hague is meeting Ukraine's interim leaders in Kiev this weekend.
He tweeted: "Have just spoken to Acting President Turchynov. I will travel to Kyiv on Sunday for talks with the new government #Ukraine."