Bulgarian Prime Minister Boyko Borisov has demanded his country be covered alongside Greece by a migration deal being negotiated between the European Union and Turkey, further complicating the already challenging talks.

"Bulgaria would not be ready to accept a partial Turkish engagement (only for the Greek islands) which will most probably lead to opening up of new migration routes," Borisov wrote in a letter to EU President Donald Tusk, a copy of which was seen by dpa on Friday.

"The Turkish land borders with EU member states - Bulgaria and Greece - and the maritime borders between Turkey and the EU, inter alia in the Black Sea, need to urgently become part of the negotiations and the agreement with Turkey," he added.

EU officials are hoping to seal the deal with Ankara at a summit in Brussels next week, but the proposed agreement has met with a flood of criticism from human rights advocates, who describe it as inhumane and likely illegal.

Worries are also mounting that migrants will soon seek alternate routes to northern Europe, after becoming stranded in Greece due to border closures in the Balkans.

Some have predicted that migrants could attempt to cross the Adriatic Sea or the Mediterranean Sea to reach Italy, but Bulgaria could also be in their sights. It shares a land and sea border with Turkey, while also being one of Greece's neighbours.

Bulgarian Defence Minister Nikolay Nenchev said on state television Friday that his country - the poorest in the EU - is ready to immediately build a fence on its border with Greece if needed.

Four hundred soldiers have already been stationed at the frontier, while Borisov has said that another 500 could soon follow.

Bulgaria is also in the process of prolonging by another 130 kilometres a 30-kilometre fence erected at its border with Turkey in 2014.

"We have repeatedly insisted on the paramount importance of protecting all EU external borders with Turkey, not only because we have to find a durable solution to the migration crisis, but because we have to guarantee the security of the EU as a whole," Borisov wrote in his letter to Tusk, which is dated Thursday.

He also raised concerns about Turkey's demand for the migration deal to include concessions on visa-free travel to Europe.

Additionally, Borisov wrote that the process should not be "calendar-driven" and called for Ankara to first implement a readmission agreement allowing for the return of illegal migrants to the country.

"I am fully aware of the urgency of the measures to be taken with respect to the refugee crisis. At the same time, we should ensure inclusiveness through participation of all member states in the decision-making process," he added.

There have been media reports that the proposed migration deal was hashed out largely by German Chancellor Angela Merkel and Turkish Prime Minister Ahmet Davutoglu.

In Greece, meanwhile, Prime Minister Alexis Tsipras said Friday that his country is working in overdrive to build accommodation for the tens of thousands of refugees stranded on its soil.

"Despite financial difficulties, each week we are building camps for 10,000 people," Tsipras said during a meeting with EU Humanitarian Aid Commissioner Christos Stylianides.

He said the humanitarian crisis in the overcrowded camp Idomeni, where more than 12,000 refugees are stranded by the blockade of Balkan borders, is a European problem that Europe should resolve.

Stylianides promised rapid EU assistance to Greece, which expects as many as 100,000 people to become stuck on its territory.

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