U.S. Secretary John Kerry says the overtures so far by Ukraine's embattled president to the country's political opposition are not enough to end the crisis.

But Kerry told reporters Friday that Ukraine's opposition has full U.S. support and if Ukrainian President Viktor Yanukovych does offer genuine cooperation, the opposition should seize the opportunity.

To date, however, Kerry said those overtures have "not reached an adequate level." Yanukovych announced Thursday he was taking an indefinite sick leave, prompting a guessing game among Ukrainians whether he was really ill or deliberately leaving the spotlight — either ahead of a crackdown or to step aside.

Kerry spoke at a news conference in Berlin before traveling to Munich for a regional security conference where he is expected to meet with Ukrainian opposition figures.