Bulgarian truck drivers have intensified a counter-blockade of border checkpoints with Greece.

With four of the crossings sealed off, Zlatograd-Xanthi and Ivaylovgrad-Kyprinos (not far from Bulgaria and Greece's borders with Turkey) are the only border crossing points open to all movement between Bulgaria and Greece as of Thursday noontime, Bulgarian border police authorities have reported.

Even at Zlatograd and Ivaylograd, only cars and buses are allowed, leaving commercial transit trucks stranded. Drivers have warned the two crossings might also be sealed off as of tomorrow.

Truck drivers in Bulgaria are staging a "counter-blockade" at checkpoints, in retaliation to the protests of Greek farmers which have severely disrupted traffic at the common border since the first blockade was set up four weeks ago.

Initially, a blockade was only observed at Kulata-Promachonas and Ilinden-Exochi border crossings, which handle most of the two-way traffic between Bulgaria and Greece.

Earlier this week, however, Bulgarian drivers extended the counter-blockade to another crossing, Kapitan Petko Voyvoda - Ormenion, adding Makaza-Nymphaea as of Thursday.

Drivers are now supported by Transport Minister Ivaylo Moskovski, who on Thursday told the Bulgarian National Television authorities were firmly behind their action.

Moskovski advised people willing to reach Greece to "consider other routes".

He accused Greek authorities of failing to temporarily lift the blockade as promised.

The minister on Wednesday unsuccessfully tried to talk officials into securing an end to the blockade.

Farmers for their part maintain the government in Athens would backtrack on plans to reform the social insurance system.