Political leaders in Greece need to show the resolve to stay in the euro zone, which will require Athens to stick to the terms of its rescue package, International Monetary Fund Managing Director Christine Lagarde said on Thursday.

"I think what we should look at is the optimal scenario where the country has the political resolve to actually observe the commitment, comply with the undertaking, stay within the zone, which seems to be the desire of the population," Lagarde said in an interview on Dutch television.

"But it goes with the effort to abide by the program which has been put in place and where the euro partners actually agreed to support the country."

Euro zone exit "would be extremely expensive and hard, and not just for Greece," Lagarde said.