Members of the Newton Teachers Association are "having conversations" about a possible strike, as union members grow increasingly frustrated at the pace of ongoing negotiations, Association President Michael Zilles said on Tuesday.

Striking is "a conversation that's happening in the buildings amongst members, certainly," Zilles said, though he emphasized that it would be a last-resort option. "You don't go on strike until you stop making progress at the table, and so we're still making progress at the table. But it is very slow."

School Committee Chair Ruth Goldman said after 15 months of negotiations, the school department and the 2,200 union members are “nearing the end and making good progress” on contract negotiations.

“At the negotiating table, I think we've had points where we've gotten a bit stuck,” Goldman said. “But I think overall, there's been a lot of movement and it's been in a fairly collaborative spirit.”

Speaking on the phone after a collective bargaining meeting Tuesday night, though, Zilles said many union members are growing increasingly exasperated with the pace of the negotiations.

“The district doesn't quite understand how frustrated our members are and how tired they are of these protracted negotiations,” Zilles said. “It has robbed people of some of the pleasures of teaching, because they just don't feel appreciated, and they're really tired of negotiations that run ... for so long.”

Zilles said the union and the school board differ on “a number of issues,” but the main sticking point is the cost of living increases. “We're still some distance apart,” Zilles said. “We moved closer in the course of the meeting, but we're still some distance apart.”

Other pivotal issues are the parental leave policy, workers' compensation and preparation time for elementary school teachers, among other issues of note across Newton’s 22 schools. The union's contract ended in August of 2018, and a one-year extension of that contract ended last September.

“There's a lot of different needs for all the different groups of people as well as the typical health care and salary issues,” Goldman said. “It's been a long and fairly involved process, but I think we are nearing the end in making good progress at this point.”

Last month, teachers in Dedham went on strike to protest an unresolved labor contract after two years of negotiating — the state’s first teacher’s strike in 12 years. The Dedham union members, who received praise from high-ranking Massachusetts politicians including U.S. Sens. Elizabeth Warren and Ed Markey and U.S. Reps. Joe Kennedy III and Ayanna Pressley, will now have to pay a fine, after breaking state law by striking. Public sector strikes are illegal in Massachusetts.

“What happened in Dedham is on everyone's mind in this district and everyone is thinking differently about that now,” Zilles said.

“To go on strike is not a decision that any union would make lightly. It's what you do when all else fails,” he said. “But I think there's a lot of frustration that this just isn't getting resolved, and people see what happened in Dedham and they say, 'Why aren't we doing that? Why can't we just resolve this?'"

Two more collective bargaining meetings are scheduled on Tuesday and Wednesday of next week, followed by two more in the week after Thanksgiving. Goldman said she’s hoping things wrap up quickly within those meetings.

“Typically contract negotiations tend to move a bit faster at the end,” she said. “I think people see that the end is in sight and we're just trying to get there fairly quickly.”

Zilles agreed, though he said he can’t predict what will happen next.

“I don't know if I'm optimistic or pessimistic. I'm just going to keep working at this and hope that we come to some sort of conclusion in the next four sessions,” he said. “I don't know how it's going to turn out, but right now we're hoping that those four do the trick.”

Correction: An earlier version of this article said the union contract ended in August 2018, but did not mention that a contract extension with the same terms was issued for the following year.