"There maybe some areas where there are tactical disagreements," said Obama at a joint press conference in the White House with Merkel, referring to the possible shift of US previous policy of not arming Ukrainian forces.

"Not just Ukraine, but the principle of territorial integrity and sovereignty is one where we are completely unified," the US president said.

However, Obama left open the prospect that if diplomatic solutions fail to work, Washington would send lethal defensive weapons to the Ukrainian forces.

"Now, it is true that if, diplomacy fails, what I've asked my team to do is to look at all options," he said.

Merkel said that military solution, though still regarded as "low prospect" by Obama, would fail to solve the crisis.

"We continue to pursue a diplomatic solution, although we have suffered a lot of setbacks," she said, hinting that a unilateral effort for Washington to arm the Ukrainian forces was not something Europe would like to see.

"But if, at a certain point in time, one has to say that a success is not possible, even if one puts every efforts into it," said Merkel, "then the United States and Europe have to sit together and try and explore further possibilities of what one can do."

Merkel's visit to Washington was the latest in her shuttle diplomacy to solve the conflict in eastern Ukraine which has already claimed more than 5,300 lives. Just returning from a surprise visit to Moscow over the last weekend, Merkel would join her counterparts from France, Russia and Ukraine for a summit in Minsk on Wednesday.

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