"I don't think it would be right for Italy to pretend not to notice when things are not getting any better," he told a conference in Florence several hours after expressing his dissatisfaction that so little had been achieved at
this week's summit in Bratislava.
EU leaders had gathered in the Slovak capital on Friday to discuss the EU's future in the wake of Britain's vote to leave the bloc, wrapping up the summit by issuing a roadmap for tackling problems such as migration, security and the faltering economy.
But Renzi said little had been achieved.
"We said more or less the same things" as in previous summits, complained the Italian leader who had been hoping for concrete action on both immigration and economic growth.
As a mark of protest, he had refused to participate in a closing press conference with French President Francois Hollande and German Chancellor Angela Merkel, saying he did not agree with them on key issues such as migration and the economy.
"I am not satisfied" with the conclusions of the summit, he later said, explaining his absence. "I cannot take part in a press conference with the German chancellor or the French president when I don't share their conclusions."
And on Saturday, he insisted that Italy would not "serve as a fig leaf" for others, in an apparent allusion to France and Germany.
For Renzi, Italy -- which has been on the frontline of the migrant crisis -- has been largely left to its own devices in coping with the influx, and the solutions it has proposed have not been taken into account.
Italy has been pushing for international agreements with African states to help close migrant routes to Europe and take back some of those arriving via Libya, in exchange for increased aid and investment.
But Renzi said the issue was not even raised at the Bratislava summit where the documents presented "didn't even mention Africa," he complained. On economic growth, Renzi reiterated his critique of Europe's adoption of austerity policies over Washington's choice of investment.
Although Italy is respecting the EU's budgetary discipline rules, it retains the right to say that such rules are "not working", Renzi said, stressing that Italy is not prepared "to pretend not to notice".
Speculation in the Italian press suggested that the reason for Renzi's bad mood was Italy's upcoming constitutional reform referendum which is scheduled for November.
Renzi has staked his political future on the outcome of the referendum on his proposed reforms of parliament and the electoral system. The vote is shaping up as a referendum on Renzi's two-and-a-half years in office and polls suggest it will be a close-run thing.