European Commission head Jean-Claude Juncker warned on Monday that a deal with Greece to unlock vital cash and allow Athens to avert default was "not yet there", just hours before a crunch debt crisis summit.

"Progress was made over the past days but we are not yet there," Juncker said as he greeted Greek Prime Minister Alexis Tsipras at EU headquarters in Brussels.

"I don't know if we will have an agreement today," he added.