The EU chief therefore spurned Tsipras's request for a telephone conversation on Saturday, with a spokesman reportedly saying that there was "nothing to discuss".
"Alexis Tsipras is my friend but friends have to observe minimal rules," Juncker said.
He added that he would like to see a Greek counter-proposal ahead of an EU summit with Latin America and Caribbean leaders next week, where he could discuss it with Tsipras.
EU President Donald Tusk also spoke out on the side of the creditors.
"If someone says I'm willing to lend money and says I hope you'll pay me back, this person is not a ruthless robber. It's not true that debtors are always moral and creditors always immoral. It's more sophisticated than what some politicians want to show today," said Tusk.
Greece's bailout agreement with the creditors expires at the end of June.
Should Greece miss its loan payments and default, many fear that this would set off a chain of events that could lead to a messy Greek exit from the euro.
Many within the ruling Syriza party, including a number of cabinet members, say the PM should call early elections rather than accept further austerity measures.
"I do not think there is much room for a positive deal with the creditors... they probably seek submission," the cabinet's chief eurosceptic, Energy Minister Panagiotis Lafazanis, told To Vima weekly.
"They want to smash the country socially and humiliate the government," he said.
AFP
