The US blasted Iran’s role in helping to “brutalize” Syria on Tuesday, questioning the Islamic Republic’s seriousness in wanting to put an end to the nearly three-year-old Syrian civil war.

US Secretary of State John Kerry is scheduled to meet with his Russian counterpart Sergey Lavrov next week to determine whether or not an invitation will be extended to the Islamic Republic to attend this month’s so-called Geneva peace conference, which is to open Jan. 22 in Montreux, Switzerland.

“At this point, Iran has done nothing but helped the [Bashar Assad] regime, help bring foreign fighters in, help the regime’s efforts to brutalize the Syrian people,” US State Department spokeswoman Jen Psaki told reporters.

“If they wanted to send a message to the world about their seriousness of having a positive outcome, there are steps they could take. There’s no indication that they have any desire or interest in taking any of these steps,” she said.

Iran has been Assad’s main ally, using the Lebanese-based Hezbollah as its proxy in the region. Fighters from the Shiite terrorist group have battled alongside Syrian regime troops, despite public opposition and increasing retribution attacks in Lebanon.

Meanwhile, members of the main Western-backed Syrian opposition group said they had postponed a vote on whether or not to attend the peace conference in Switzerland.

The Syrian National Coalition had been expected to decide Tuesday whether or not to attend the conference.

But coalition officials said that amid sharp disagreement over the issue, the vote was postponed until at least the middle of next week. That would be less than a week before the conference. The postponement of a vote whose outcome is uncertain adds to doubts about the conference whose prospects already look shaky.

Hadi Al Bahra, a member of the coalition’s political committee, said the vote could happen as early as the Jan. 15. Al Bahra said that many members are angry that the government of President Assad has not agreed to the principles from an earlier peace conference that called for a transitional government in Syria. Assad’s government has said the president will not surrender power and may run again in elections due later this year.

The possibility that Iran might join the talks also has provoked disagreement within the coalition. Though Iran was not among the recipients of invitations to the conference sent by UN Secretary-General Ban Ki-moon Monday, his spokesman has said that Ban favors inviting Iran.

Abdulrahman Alhaj, a member of the Syrian National Council, which is an influential block within the coalition, said that sharp disagreement over attending the talks could fracture the coalition.

“If that happens, there won’t be a Geneva conference,” said Alhaj, who is not a member of the coalition. “We need a credible, united opposition to go to Geneva.”