The Greek government is entering a new round of negotiations with the country's creditors over the course of the latest bailout program, amid angry union reactions to proposed pension reforms that are a key part of the talks.

Representatives from Greece's European creditors and the International Monetary Fund were due to meet Finance Minister Euclid Tsakalotos on Monday. Greek officials say there will be a break in the talks at the end of this week, but it is unclear how long that will last.

In return for money from the country's third, around 80-billion-euro ($87 billion) bailout, agreed last summer, Greece's left-led government has to meet a series of conditions, from reducing spending to enacting wide-ranging economic reforms. Meeting the targets set will also open up discussions over debt relief — after years of savage austerity, Greece's debt burden stands at around 175 percent of its annual GDP.

One of the main reforms it has to undertake next is to overhaul the pensions system, which the government has admitted is in a parlous state.

But unions representing groups as disparate as farmers, police, fishermen and lawyers fiercely oppose the draft reforms, saying they will end up paying most of their income in taxes and social security contributions, and plan a string of protests and strikes this week.

Farmers used scores of tractors Monday to indefinitely block two border crossings with Turkey and Bulgaria, stopping lorries but allowing all other traffic to pass. Other crossings with Turkey and Bulgaria were being sporadically closed, as were highways across Greece.

Unions have called a general strike for Thursday. Journalists, public transport employees, lawyers and notaries public will also be walking off the job in coming days.