Jeroen Dijsselbloem, who heads the group of euro zone finance ministers, was speaking to the Dutch parliament, where a majority of lawmakers called for no new financial support for Greece.
"An agreement is an agreement," Dijsselbloem said. "That means that only within the programme, measures could be exchanged for other measures."
He said any changes would have to be approved by the troika of lenders comprising the European Commission, the European Central Bank and the International Monetary Fund.
"Then you can change the programme, you can fill the programme in differently. But support without a programme, support without further progress on reforms is unthinkable," he said.
Dijsselbloem was speaking before heading to Brussels for a eurogroup meeting on Greece.
Greek Finance Minister Yanis Varoufakis will present his government's demands for an end to its international bailout and a transition to a new debt restructuring deal.
At his first appearance before the group, Yaroufakis will seek a "bridge agreement" until a full settlement can be reached in June.
"We have the joint ambition to come to a solution, and I do think that it's possible," Dijsselbloem said. "The alternative measures will always have to be vetted, tested by the Troika, whether you call it the Troika or not."
(Reporting By Toby Sterling and Thomas Escritt; Editing by Anthony Deutsch and John Stonestreet)
By Toby Sterling
