Iran supports a Russian-led effort to impose Syrian government control over the south of Syria, a senior Iranian security official was quoted as saying on Saturday, amid reports that Damascus is preparing a major military offensive in the area.
Russia said last week that only Syrian army troops should be on the country's southern border with Jordan and Israel.
Syrian government forces, in their strongest position since the early months of the seven-year conflict, have driven rebels out of all territory near the capital Damascus this year.
For weeks there have been reports that the government's next target would be the zone in the south, one of only two large areas left in the hands of fighters seeking to topple President Bashar al-Assad.
Washington says any offensive in the area would violate a ceasefire it has jointly sponsored with Moscow for that part of Syria, and has warned it would take "firm measures" in response.
Rebels control stretches of southwest Syria, bordering the Israeli-occupied Golan Heights, while Syrian army troops and allied Iran-backed militias hold nearby territory.
"We strongly support Russian efforts to drive terrorists out of the Syria-Jordan border and to bring the area under Syrian army control," the secretary of Iran's Supreme National Security Council, Ali Shamkhani, told the Shargh Daily.
He also repeated Tehran's denial that it has military advisors in that part of Syria. Israel says Iranians are operating in the area near its border and has called for Moscow to keep Iranian forces and their allies away from the frontier.
"We have said before that Iranian military advisers are not present in southern Syria and have not participated in recent operations," Shamkhani said.
Russian Defence Minister Sergei Shoigu on Thursday discussed the so-called de-escalation zone in southern Syria, where the United States and Russia sponsor a truce, with Israeli Defence Minister Avigdor Lieberman in Moscow.