The founding members of Mercosur - Argentina, Brazil, Paraguay and Uruguay - gave Venezuela an ultimatum on Tuesday amid concern over the country's human rights record, restrictions on the political opposition and failure to comply with trade rules.

The four countries also said Venezuela would be blocked from taking over Mercosur's rotating presidency and instead a commission with one member from each of the founding states would take over the role.

Brazil's foreign ministry said in a statement that the decision was designed to "preserve and strengthen Mercosur."

The ultimatum is another blow to an increasingly isolated Venezuela as it grapples with dual economic and political crises.

"Having that Mercosur membership is seen as important by the Venezuelan regime to show its population that it is integrated in the region," said Oliver Stuenkel, professor of international relations at Sao Paulo-based think tank FGV. "This will certainly add pressure, but is unlikely to change things in Venezuela."

The founding members did not specify what steps Venezuela would have to take to comply with the membership requirements. Brazil has said Venezuela had met only about 30 percent of them.

Oil rich Venezuela was accepted into the trade bloc in 2012 with a four-year timeline to meet all requirements.

cw/kms (AP, Reuters)