The leaders of the Group of Seven major economies agreed on Saturday to swiftly impose further sanctions on Russia over the Ukraine crisis, and the United States could unveil its new punitive measures as early as Monday, officials said.

"We believe that these sanctions will have a significant impact," U.S. Deputy National Security Advisor for Strategic Communication Ben Rhodes said on Saturday.

President Barack Obama spoke to four European leaders on Friday night about new sanctions against Russia, stressing the need to move quickly, Rhodes told reporters aboard Air Force One, as Obama flew from South Korea to Malaysia on an Asian trip.

"There was quick agreement about the need to move forward with a sequences of steps," said Rhodes.

"The sequence that was agreed to in the leaders' call last night was that the G7 would express its support for targeted sanctions against Russia. The U.S. and EU would move out on their own."

A senior U.S. official said each G7 country would decide which targeted sanctions to implement and while the measures would be coordinated they would not necessarily be identical.

Rhodes said sanctions are possible on individuals or companies with influence in specific sectors of the Russian economy such as energy and banking.

He said such sanctions have consequences "when you start to get at the cronies, the individuals who control a large part of the Russian economy, you are imposing a larger economic impact than sanctioning an individual."

The new sanctions are intended to punish Russia for failing to comply with an international agreement to help defuse the Ukraine crisis, according to a statement from G7 leaders released by the White House.

"Given the urgency of securing the opportunity for a successful and peaceful democratic vote next month in Ukraine's presidential elections, we have committed to act urgently to intensify targeted sanctions and measures to increase the costs of Russia's actions," the statement said.

Washington will announce its list as early as Monday, senior U.S. officials said. Sources familiar with the matter said the U.S. list of individuals expected to include "cronies" of Russian President Vladimir Putin.

The G7 leaders told Russia that "the door remains open to a diplomatic resolution of this crisis" on the basis of the Geneva accord and urged Moscow to take that path.

But they also warned that "we continue to prepare to move to broader, coordinated sanctions, including sectoral measures, should circumstances warrant."

"Everybody understood that if Russian troops on the border invade Ukraine then sector sanctions will be a response. We need to have a spectrum of sanctions we can impose," said Rhodes.

"We understand that there is unease about the economic consequences of increased sanctions on a large economy like Russia. Our response is that the long-term consequences of allowing Russia to engage in this type of destabilising activity is going to carry its own type of economic costs." – Reuters