The two NGOs sent the letter on behalf of Forum 2020 over the current migrant crisis on the Greek border and the announcement that Croatia will use all available means to help protect the EU's external borders, which could result in the army being deployed on the Croatian border.
That, Forum 2020 believes, is not in line with Plenkovic's role as the prime minister of the country currently chairing the Council of the EU.
"Given that in your announcements of the presidency, you said the Croatian government would push for a reform of the Common European Asylum System and better cooperation with third countries, which includes Southeast Europe, Forum 2020 believes your first step in the context of the latest situation should be in that direction," Forum 2020 says in the letter to Plenkovic.
Forum 2020 is an informal platform of civil society organisations formed on the occasion of Croatia's EU presidency in the first half of this year.
It recalls that Plenkovic said in 2016 that reforming the CEAS was an important step towards a more equitable solution to the refugee crisis by sharing responsibility and solidarity of all member states in the reception of migrants.
Forum 2020 says Plenkovic has not pushed for that policy actively and sovereignly over the past four years but allowed Croatia to take on from the EU "the heavy burden... of the inhumane treatment of refugees and migrants and violations of their human rights."
The CEAS must be based on human rights, have clear goals and remove the dysfunctions of the current system, and its reform must protect and strengthen, not reduce the rights of asylum seekers and refugees, Forum 2020 says.
The current instability and insecurity on the EU's external border is a consequence of inadequate EU policies, primarily harmful agreements with Turkey and Libya, which violate human rights and build their positions on blackmail, undermining respect for human dignity and the rule of law which the EU has the duty to protect, the platform says.