Non-tenured instructors at the University of Illinois at Urbana-Champaign began a five-day strike Thursday.

The walkout is the second strike held by the Non-Tenure Faculty Coalition Local 6546 this month as part of its call for a labor contract, which would be the union's first with the university. The union held a two-day strike last week.

Contract negotiations between the union and university officials have been ongoing since October 2014. The two sides meet with a federal mediator on Wednesday, but the negotiations failed to produce an agreement. As a result, the union voted Wednesday night to stage a five-day walkout through May 4.

According to university officials, additional contract negotiation sessions have been scheduled for Friday and Tuesday.

Non-Tenure Faculty Coalition Local 6546 President Shawn Gilmore wrote an email to union members Wednesday.

"This strike can be suspended at any time if the administration decides to engage in meaningful negotiations with us that will lead to a settled contract," he said in the email, according to The News-Gazette.

Amid contract negotiations, union members held a "work-in" at the campus on Tuesday and Wednesday to highlight the important work they do at the school. They graded papers, held office hours and conducted research at the university's administration building.

There are about 500 workers represented by the union on U of I's campus.

The educators are seeking long-term contracts based on an individual's number of years teaching at the school. University officials, who back the idea of longer contracts, want the deals administered based on merit and at the discretion of the department in which the instructor teaches.

In a campus email, interim Chancellor Barbara Wilson said the university has made revisions to its original contract offers.

"We made progress on several issues," Wilson said, adding that the university officials "regret the disruption another work stoppage will cause our students but we remain committed to arriving at a contract that is in the best interest of our university."