BARACK OBAMA has warned Russia's oil and gas businesses may be the next to be targeted for sanctions if the country's troops move further into Ukraine.
The US President said after talks with European Union officials in Brussels that they are co-ordinating the potential for further measures following the annexation of Crimea.
Any such move against Russia's energy sector would potentially affect one of the world's largest companies, Gazprom.
The US has already imposed visa bans on a number of high profile Russians linked to President Vladimir Putin, and frozen the St Petersburg-based Rossiya bank 'out of the Dollar'.
Mr Obama said: "What we are now doing is co-ordinating around the potential for additional, deeper sanctions should Russia move forward and engage in further incursions into Ukraine.
"I think energy is obviously a central focus of our efforts and we have to consider it very strongly."
Mr Obama also urged Europe to work towards diversifying its energy sources, in addition to asking Washington for help to offset any shortfalls from Russia.
He added: "I think it is useful for Europe to look at its own energy assets as well as how the United States can supply additional energy assets."
Ukraine's government yesterday announced it is to hike the price of domestic gas for customers by 50% from May 1. Domestic consumers in Ukraine have long received supplies of gas for home central heating at subsidised prices in a hangover from the Soviet era.
The announcement will please the International Monetary Fund, which is expected to announce a $15 billion (£9bn) bailout as early as today.
David Cameron also 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 at Downing Street.
He pledged to continue speaking out about the "unacceptability" of Russian behaviour.
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.
"We will continue to send very tough messages about the unacceptability of Russian behaviour."
Mr Cameron also told MPs yesterday the failure to stand up to President Vladimir Putin over the Crimea would lead to further "unacceptable behaviour" by Russia in other neighbouring countries.
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,
He said it "could help make a significant contribution to de-escalate the situation" and would provide impartial and independent verification of what is happening on the ground.
l Russians have begun an internet campaign of tongue-in-cheek "sanctions" against President Obama.
"Barack Obama as well as members of the US administration, Senate and Congress are forbidden from wishing me a happy birthday," Instagram user kos77 posted along with a photo of a frowning Mr Obama.