U.S. Secretary of State John Kerry on Sunday called for Russia to pull back its forces from the borders of Ukraine, and said any talks on the country's future must include Kiev's leaders.

"We will not accept a path forward where the legitimate government of Ukraine is not at the table. This principle is clear. No decisions about Ukraine without Ukraine," Kerry told reporters.

The United States has sent its top general in Europe back to the continent early, amid what the Pentagon on Sunday called Russia's "lack of transparency" over the Ukraine crisis.

General Philip Breedlove, NATO's supreme allied commander Europe and the U.S. military's European Command chief, returned to Europe on Saturday from a visit in Washington, where he had been due to testify before Congress.

U.S. Secretary of State John Kerry meets his Russian counterpart on Sunday hoping to ease global tensions over the Kremlin's annexation of Crimea and sudden buildup of troops near Ukraine.

The latest attempt to resolve the worst East-West standoff in the post-Cold War era follows a sudden call placed by Vladimir Putin on Friday to U.S. President Barack Obama -- the first by the Russian leader since he sent troops into Ukraine's Black Sea peninsula in response to the February 22 fall in Kiev of a pro-Kremlin regime.

Russian President Vladimir Putin wants to carry on the frequent "tough but constructive" contacts he has had with Chancellor Angela Merkel on the Ukraine crisis, his spokesman has told German TV.

Dmitry Peskov told ZDF television that the telephone talks between Putin and Merkel were worth their "weight in gold", according to excerpts released ahead of broadcast on "Berlin Direkt" later Sunday.

Russian Foreign Minister Sergei Lavrov on Sunday called on Western powers to back a proposal for Ukraine's Russian-speaking regions to have greater powers in a "federal" structure.

"If our Western partners are ready, then Russia, the United States and the European Union could form a support group on Ukraine and formulate shared appeals to those now in power in Kiev," Lavrov said in an interview with Russian state television.

NATO chief Anders Fogh Rasmussen said Sunday that the Ukraine crisis underscored the need to protect the right of nations to map out their own future.

In an op-ed entitled "The right to choose", the outgoing secretary general called Russia's actions in Crimea a violation of Ukraine's territorial integrity and sovereignty, which undermined the rule of international law.

As the rest of Europe prepares to set their clocks one hour forward for summer on Sunday, residents of Crimea braced for a two hour jump into the timezone of their new masters in Moscow.

To seal Russia's takeover of the formerly Ukrainian peninsula, a symbolic ceremony will be held to move the clocks forward at the railway station in the main city of Simferopol.

Russia's foreign ministry said Saturday that Moscow and Washington still had significant differences over ways to settle the Ukrainian crisis despite ongoing consultations about the biggest East-West standoff since the Cold War.

"There is no single plan," RIA Novosti quoted Russia's Deputy Foreign Minister Sergei Ryabkov as saying. "We have differing views of the situation. Our discussions involve an exchange of ideas, but one cannot say that we have some sort of single approach."

U.S. Secretary of State John Kerry will meet Russian Foreign Minister Sergei Lavrov for talks on the Ukraine crisis on Sunday, State Department spokeswoman Jen Psaki told reporters.

"The meeting is in Paris tomorrow evening," she said after Kerry left Riyadh early on Saturday, due to fly home to Washington via a refueling stop in Ireland. But Kerry decided mid-flight to go to Paris instead.

Boxer turned opposition leader Vitali Klitschko on Saturday dropped out of Ukraine's snap presidential polls to help the candidacy of a charismatic tycoon who made a fortune selling chocolates and backs closer Western ties.

The towering sports star's decision leaves the crisis-hit nation of 46 million with two clear frontrunners in the May 25 vote -- business baron Petro Poroshenko and the highly divisive and corruption-stained opposition leader Yulia Tymoshenko.