AFP- After a gap of 27 years, direct flights between Iran and Serbia resumed on Saturday, when an IranAir jet touched down at Belgrade's Nikola Tesla airport, Serbian media reported.

IranAir is offering a direct service between Tehran and Belgrade twice weekly, with all flights fully booked until the end of the summer, the reports said.

But the service, which was launched following a visa liberalisation agreement between the two countries, has raised fears it could open up a new migrant route for those seeking to stay in the EU illegally, a Serbian charity has warned.

A second Iranian carrier, Qeshm Air, is also planning to launch a service between the two capitals starting from March 19.

In August 2017, Iran and Serbia agreed to liberalise visas for travel between the two countries, sparking a surge in interest on the part of Iranians.

According to the Serbian non-governmental refugee support project Info Park, some Iranians are using the visa liberalisation agreement to come to Europe and stay there illegally as migrants.

Last month, Info Park said a number of Iranians had arrived in Belgrade legally as tourists but had not returned home, proceeding instead to EU countries, notably France and Germany.

"Although they entered Serbia as tourists, interviews have revealed that many Iranians use their stay in Belgrade to establish connections with smugglers, who will transfer them to their desired destination, across the borders of Croatia, Bosnia and Herzegovina, Hungary or Romania," Info Park said at the time.

It said those interviewed said they were leaving for a variety of reasons including fears for their rights and freedoms, particularly linked to their political, religious or sexual orientation.

- New migrant route? -

Info Park said an estimated 600 Iranians could arrive in Belgrade in a single week, based on the current transport capacity.

"Seeing as most of these new-arrivals do not intend to return, the migration systems in Europe must recalibrate for this new route and demographic among the migrant populaces," the NGO said.

Serbian Trade Minister Rasim Ljajic said authorities in the two countries would investigate possible abuses of the visa liberalisation agreement.

Checks would be reinforced in Tehran and various bilateral deals would be signed with the aim of clamping down on illegal migration, the ministry said.

Since the visa requirements were lifted, around 7,000 Iranians have travelled to Serbia, of which 485 have applied for asylum, the ministry said.

Serbia was one of the countries on the so-called Balkans route to western Europe, with hundreds of thousands of migrants passing through until the route was shut down at the start of 2016.

In January, the UN's Refugee Agency, UNHCR, said there were around 4,000 migrants currently in Serbia.