(Carolyn Kaster/AP Photo)

TOKYO - President Obama warned more U.S. sanctions on Russia are "teed up and ready to go," signaling that if Russia does not reverse course in eastern Ukraine it will face additional consequences "in days not weeks."

But even as he threatened further sanctions, Obama acknowledged it is "entirely possible" that new economic penalties will not work to change Russian President Vladimir Putin's calculus on Ukraine.

"So far the evidence doesn't make me hopeful," Obama said. "Assuming they don't follow through, then we'll follow through on what we said… which is tighter consequences on the Russians."

Further sanctions, he said, are ready go.

"We have been preparing for the prospect that we have to engage in further sanctions. Those are teed up," he said, adding that they require "technical work" and coordination with other countries.

"The fact I haven't announced them yet, doesn't mean they aren't teed up and ready to go," the president said.

Obama also had tough words for China, a country whose "peaceful rise" he has celebrated but will not visit on this trip.

Obama made it clear that the U.S. security commitment to Japan extends to the Senakaku Islands, claimed by both China and Japan but currently administered by Japan. The Japanese fear China could move to militarily take over the islands.

"Let me reiterate that our commitment to Japan's security is absolute," he said, "and covers all areas, including the Senkaku Islands."

A few minutes later, President Obama bristled at the suggestion that he was drawing yet another red line as he had done with Syria on chemical weapons and Russia on Crimea.

"The treaty between the U.S. and Japan preceded my birth. So obviously this isn't a red line I'm drawing," Obama said. "This is an interpretation that has stretched multiple administrations about alliance. No shift in position, no red line that's been drawn. Simply applying the treaty."

As for Syria, the president sought to portray his red line policy a success, saying that 87 percent of Syria's chemical weapons stockpiles have been removed.

ABC"s Devin Dwyer contributed to this report.