WARSAW, Poland (AP) — The Polish government offered direct talks with teachers unions Tuesday as a nationwide strike that has closed thousands of schools entered its second day and threatened upcoming graduation exams.

The head of the main teachers' union, ZNP, said its representatives will attend the talks.

Slawomir Broniarz earlier called for a mediator to help end the bitter pay standoff with the government. The government responded with an invitation to direct talks later Tuesday, and said it was prepared immediately to sign an offer it made previously, which ZNP and another union had rejected as unsatisfactory.

Meanwhile, school directors were asking for help from retired teachers, religion teachers and those not on strike to press ahead with the middle school exams, due to start Wednesday.

Many parents expressed support for the strike and offered help in arranging classrooms for the test.

The indefinite strike started Monday after teachers' pay negotiations with the government failed. Monthly net earnings for chronically underpaid teachers range from 1,800 zlotys to 3,000 zlotys ($470 to $780).

They want an urgent 30% raise, arguing that the government has found substantial new funds for families, pensioners and businesses. The government is offering to speed up planned increases of some 15% and a new remuneration system, which would substantially raise salaries but would also increase the number of lessons teachers teach to 24 from the current 18.

Broniarz said he was urgently waiting for negotiations to resume, through a mediator, to find a solution and "put out this fire."

Deputy Prime Minister Beata Szydlo responded with an invitation for talks later Tuesday, for the existing pay offer to be signed and for discussion on changing the whole system of remuneration.

Broniarz said that over 74% of more than 20,000 schools nationwide were closed Monday. According to the Education Ministry it was just over 48% of schools and education centers.

If it continues, the strike could prevent students from taking their middle school graduation exams starting Wednesday and elementary school exams next week.