By Mick Krever, CNN

A phone call between former Ukrainian President Viktor Yanukovych and Russian President Vladimir Putin may have been the deciding factor in the Ukrainian leader changing his "attitude" towards the protests in Kiev, Polish Foreign Minister Radoslaw Sikorski told CNN’s Christiane Amanpour on Wednesday.

Sikorski was intimately involved in the negotiations that brought a truce between Yanukovych and the Ukrainian opposition, and gave Amanpour an insider’s view of the talks.

“President Yanukovych left us several times to talk to [U.S.] Vice President Biden, [German] Chancellor Merkel, and indeed President Putin,” Sikorski said. “One of the breakthroughs was when we said, ‘Well look, Mr. President, you have to declare to the opposition by when you agree for new presidential elections to be held, by when do you intend to shorten your term of office.’”

“He was very reluctant, as you might imagine,” Sikorski said. “His attitude changed after one of the conversations, we think, with President Putin.”

Polish FM on Ukraine future Click to watch video

Sikorski told Amanpour he was “not privy” to the conversation, and does not know what Putin said to convince Yanukovych so suddenly.

The Ukrainian opposition leader Arseniy Yatsenyuk was named as the nominee for Ukraine interim prime minister on Wednesday, just days after Yanukovych fled the capital.

Sikorski called Yatsenyuk, a former foreign minister, “experienced,” “respected,” and “very competent.”

He is “certainly someone we can do business with, and someone who will have the confidence of the West, but also the ability to speak to Russia, which is very important.”

Ukraine’s interim government will face a slew of problems, including unrest in the Russia-sympathetic east and south of the country, and a financial situation that the acting president described Tuesday as “sliding into the abyss.”

“Ukraine is definitely in a crisis. Let’s hope it doesn’t deepen.”

“But Ukraine now has a chance to stabilize its institutions, to start reform, get the second tranche of IMF money, elect a new president, sign an association agreement with Europe, and become a better country.”