Iran said Thursday that Saudi Arabia should not bring "terrorists in a new mask" to the negotiation table for preliminary peace talks between the Syrian government and opposition groups.

Syria has been engulfed in a near six-year-long civil war that has left 250,000 dead as a number of opposition groups supported by outside countries fight to overthrow President Bashar al-Assad. The United Nations Security Council adopted a Jan. 30 deadline for peace negotiations to begin between Assad, who is backed by Iran and Russia, and opposition forces, who are backed by Saudi Arabia and the U.S. Talks were set to begin on Monday in Geneva but were delayed until Friday due to disagreements over which rebel groups should be invited.

"Terrorists with a new mask should not sit down at a negotiating table with the representatives of the Syrian authorities," Deputy Iranian Foreign Minister Hossein Amir-Abdollahian told a news conference during a visit to Russia, reported Reuters. "This is the most important condition."

Amir-Abdollahian asked Saudi Arabia to stop meddling in Syrian affairs because it increases tensions in the region, and he accused the country of attempting to wield more influence at the negotiations by adding "terrorists" to the opposition negotiation list.

"We believe that Saudi Arabia's insistence on including recognizable terrorists ... in one list or another, is certainly not a constructive action on its part," Amir-Abdollahian said.

He added: "In regard to the opposition groups that will participate at the Geneva negotiations, we have presented to U.N. Special Envoy for Syria [Steffan de Mistura] some Syrians who think they may be useful at these talks," according to Sputnik News.

Organizers say that the talks will not be direct peace negotiations, but proximity discussions designed to lay the groundwork for later peace talks, according to Voice of America.

Haitham Manna, co-president of an Arab and Kurdish opposition group, said Thursday that he doesn't believe that the talks will begin Friday due to "technical reasons," while another opposition figure said that it will be difficult to begin talks Friday because many in the opposition still need to obtain visas and make reservations to travel to Geneva, reported the Associated Press.

Another opposition group, backed by Saudi Arabia, says that before it agrees to participate, the Syrian government should stop bombing rebel-held areas.

The Syrian government, with the support of Russian airstrikes and Iranian militia forces, has been gaining ground against the rebels, capturing the key southern town of Sheikh Maskin on Monday and cutting off rebel factions from valuable supply lines, reported Al Jazeera.