The new thresholds for action were spelled out at a joint press conference, following a Group of Seven world leader summit that was rearranged to exclude Russian President Vladimir Putin after his aggressive moves in Ukraine. The United States and Europe also have imposed economic sanctions in response.
To avoid even harsher sanctions, Cameron said, Putin must meet three conditions: Recognize Petro Poroshenko's election as the new leader in Kiev; stop arms from crossing the border; and cease support for pro-Russian separatist groups concentrated in eastern Ukraine.
"If these things don't happen, then sectoral sanctions will follow," Cameron said, standing before a row of United States and British flags. "The next month will be vital in judging if President Putin has taken these steps. And that is what I will urge President Putin to do when I meet him later today."
Obama said the G-7 leaders unanimously agree with the steps Cameron outlined. But they were not so explicit in written statements issued after two days of meetings, and an Obama aide later described the potential sanctions in different terms than Cameron.
"If Mr. Putin takes those steps, then it is possible for us to begin to rebuild trust between Russia and its neighbors and Europe," Obama said. "We will have a chance to see what Mr. Putin does over the next two, three, four weeks, and if he remains on the current course, then we've already indicated the kinds of actions that we're prepared to take."
Obama acknowledged that so-called sectoral sanctions, which would hit key sectors of Russia's economy, could have a bigger impact across Europe because of economic ties to Russia, and said he didn't necessarily expect all European countries to agree on them. But, he said, "it's important to take individual countries' sensitivities in mind and make sure that everybody is ponying up."
"My hope is that we don't have to exercise them because Mr. Putin's made some better decisions," Obama said.
Obama foreign policy adviser Ben Rhodes later said it wasn't certain that sectoral sanctions would be the punishment of choice. "We'll be calibrating those based on what the situation is and that sectoral sanctions are in the tool kit," Rhodes said.
Rhodes was speaking to reporters onboard Air Force One en route to Paris, where Obama later arrived for dinner with French President Francois Hollande. Hollande planned to dine a second time later in the evening with Putin, so the U.S. and Russian leaders would not have to cross paths.
Putin's meetings with Cameron, Hollande and German Chancellor Angela Merkel illustrate how Obama and European leaders are taking different strategies for dealing with the Russian leader, after trying to isolate him over his moves in Ukraine – including Russia's annexation of the Crimean Peninsula.
Obama was not meeting with Putin one-on-one, but said they may have an opportunity to talk when they both attend events commemorating the 70th anniversary of the D-Day invasion Friday in Normandy.
"There is a path on which Russia has the capacity to engage directly with President Poroshenko now. He should take it," Obama said his message to Putin would be. "If he does not, if he continues a strategy of undermining the sovereignty of Ukraine, then we have no choice but to respond.
Obama said he thought the fact Putin didn't immediately denounce the outcome of Ukraine's election last month offers hope he's moving in a different direction. "But I think we have to see what he does and not what he says," he added
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