TOKYO — Warning Russia that new economic sanctions are "teed up," President Barack Obama accused Moscow of failing to live up to an agreement last week to ease tensions in eastern Ukraine.

Still, he cautioned that the United States needs to secure the support of allies to ensure that additional economic pressure is even applied. He conceded that new sanctions may not change Russian President Vladimir Putin's geopolitical calculations.

"There are some things the United States can do alone but ultimately it's going to have to be a joint effort, a collective effort," Obama said during a press conference with Japanese Prime Minister Shinzo Abe.

Obama's comments underscored the difficulties he faces in devising a response to Russia's aggressive moves on Ukraine's eastern border and the growing unrest in the country driven by pro-Russian insurgents. He did not put a timeline on when sanctions could be applied, saying only it was a matter of days, not weeks.

Obama complained that militias and armed men continue to take over government buildings in Ukraine in defiance of Ukrainian authorities. Pro-Russian insurgents have been especially active in eastern Ukraine in the aftermath of Russia's annexation of Ukraine's Crimean peninsula. Under an agreement struck last week in Geneva, Russia had agreed to take steps to defuse the tensions.

"So far we have seen them not abide by the spirit or the letter of the agreement in Geneva," Obama said. If that continues, he said, "there will be further consequences and we will ramp up further sanctions."

Obama's caution came in the wake of a warning by the Russian foreign minister that attacks on Russian citizens or interests in Ukraine would bring a firm response. Foreign Minister Sergey Lavrov compared the circumstances to those that triggered the war with Georgia in 2008.

By acknowledging that he still needed cooperation from allies to impose new sanctions, Obama laid bare one of the key obstacles to presenting a united front against Russia. Many European countries rely on Russian energy and fear that increased pressure on Moscow could hurt their own economies.

"It's important to emphasize that throughout this process our goal has been to change Mr. Putin's calculus, that our preference is to resolve this diplomatically, that sanctions hurt Russia more than anybody else but they are disruptive to the global economy," Obama said.

In Russia, Lavrov on Wednesday declared that attacks on Russian citizens are attacks against the Russian federation. His comments came day after Ukraine announced it was re-launching a campaign against pro-Russia insurgents occupying government facilities in the mostly Russian-speaking east.

"If we were attacked we could certainly respond," Lavrov said.