BRUSSELS -- With the backing of other world leaders, President Barack Obama effectively set a one-month deadline for Moscow to reverse its intervention in Ukraine and help quash a pro-Russian separatist uprising or else he said it would face international sanctions far more severe than any it had endured so far.
Mr. Obama and other leaders of seven major democracies meeting in Brussels demanded that Russian President Vladimir V. Putin recognize and negotiate directly with the newly elected Ukraine leader, stop the flow of fighters and arms across the border and press separatists to disarm, relinquish seized public buildings and join talks with the central authorities in Kiev.
"Russia continues to have a responsibility to convince them to end their violence, lay down their weapons and enter into a dialogue with the Ukrainian government," Mr. Obama said at a news conference alongside British Prime Minister David Cameron after meeting leaders of the Group of 7 industrial powers. "On the other hand, if Russia's provocations continue, it's clear from our discussions here that the G-7 nations are ready to impose additional costs on Russia."
For the first time, Mr. Obama laid out a time frame, saying the process could not drag out.
"We will have a chance to see what Mr. Putin does over the next two, three, four weeks," he said, "and if he remains on the current course, then we've already indicated what kinds of actions that we're prepared to take."
So far, the United States and European allies have imposed only limited sanctions, aimed at individual Russians and a handful of their companies, in retaliation for the Russian annexation of Crimea and the violence in eastern Ukraine. The next stage would be broader, cutting off dealings with sectors of the economy such as finance and energy.
European leaders have resisted such an escalation at least in part because their countries have much deeper economic ties to Russia than the United States does.
Germany receives about one-third of its natural gas from Russia. Britain has extensive banking ties. France does a thriving arms business with Moscow.
Mr. Obama and German Chancellor Angela Merkel had threatened to impose sectoral sanctions if Russia disrupted the May 25 election in Ukraine that produced Petro Poroshenko as the new president.
Pro-Russian separatists worked to stop the voting in the east, but the United States and European leaders chose not to see the problems as grave enough to merit following through on the threat.
U.S. and European officials are hoping Mr. Putin's decision to pull some troops back from the border signals that he wants to defuse the confrontation, either because of damage done to his own economy or because he did not find as much popular support for joining Russia in eastern Ukraine as he did in Crimea.
In a small sign of a thaw, Moscow announced Thursday that its ambassador to Kiev, Mikhail Zurabov, would return to Ukraine in time to attend Saturday's inauguration of Mr. Poroshenko.
Mr. Zurabov was withdrawn in February, when Russia objected to the overthrow of President Viktor Yanukovych.
It remained unclear what the West would do if Russia largely left matters as they were, neither escalating the crisis nor reining in separatists.
France has said that as things stood, it intended to proceed with a $1.6 billion sale of warships to Russia; Mr. Obama took issue with that again Thursday before leaving Brussels for Paris to dine with French President François Hollande.
The Group of 7's summit meeting was the first in two decades to exclude Russia, which began attending as a guest in 1993 and joined as a full-fledged member in 1998, but was suspended after annexing Crimea.
Even so, Mr. Hollande, Mr. Cameron and Ms. Merkel all scheduled individual meetings with Mr. Putin for Thursday or today.
Mr. Hollande, in fact, hosted him for a light supper after dining with Mr. Obama.