The UN envoy for Syria, Staffan de Mistura, was in Riyadh Tuesday for talks with diplomats and Syrian opposition representatives on efforts to end the nearly five-year war, an Arab source said.

His visit comes amid heightened tensions between Saudi Arabia, the main Sunni power in the region, and Shia-dominated Iran over Riyadh's execution of a prominent Shia cleric and dissident.

There are concerns that the row could derail growing efforts to end the conflict, as Riyadh backs rebels fighting to overthrow President Bashar al-Assad and Iran supports the Syrian president's regime.

De Mistura is also expected in Iran later this week and in Damascus on Saturday, according UN sources.

Early Tuesday, he met ambassadors of countries that were involved in discussions on Syria in Vienna in October and November, in which both Tehran and Riyadh took part, the Arab source told AFP.

Those talks broke ground by drawing up a roadmap, unanimously adopted by the UN Security Council on December 19. The plan foresees talks between the different sides on January 25, establishment of a transitional government within six months and elections within 18 months.

De Mistura will also meet Syrian opposition representatives in Riyadh to discuss the date for peace negotiations, an agenda for the talks and the names of members of their delegation, the source said.

Saudi Arabia cut diplomatic ties with Iran Sunday after protesters ransacked the Saudi embassy in Tehran and consulate in Mashhad over the execution of Shia cleric Nimr al-Nimr.

UN Secretary General Ban Ki-moon spoke by telephone with the Saudi and Iranian foreign ministers to urge them to "avoid any actions that could further exacerbate the situation," spokesman Stephane Dujarric said.

"A breakdown of relations between Riyadh and Tehran could have very serious consequences for the region," said Dujarric.