At the meeting in the Uruguayan capital Montevideo, the leaders sought to pressure their host over plans for go-it-alone trade deals with China and the Trans-Pacific Partnership (TPP).
"The solution is not for each one of us just to do our own thing," said Argentine President Alberto Fernandez, chiding Uruguay.
One of the smallest economies in South America, Uruguay has been trying for years to conclude unilateral trade agreements that it considers more beneficial than Mercosur.
Of course, if we go in a group it is much better," said Uruguayan President Luis Lacalle Pou.
Paraguayan President Mario Abdo Benítez said Mercosur integration was "fundamental and strategic" but indicated flexibility on a demand Uruguay not move forward on trade deals without informing the others.