Russian President Vladimir Putin is in Tehran for talks with Iran's leadership on the Syria crisis, the landmark 2015 international nuclear agreement, and energy cooperation.
Upon arrival, Putin went straight into talks with President Hassan Rouhani.
During his one-day visit, Putin was also to meet with Supreme Leader Ayatollah Ali Khamenei, as well as participate in a trilateral summit with Rohani and Azerbaijani President llham Aliyev.
Aliyev, who hosted a similar three-way meeting in August 2016, arrived in Tehran earlier on November 1 and held a first separate meeting with Rohani.
Iran, Russia, and Azerbaijan share the Caspian Sea, and railway and road projects are expected to be discussed at their meeting.
Kremlin spokesman Dmitry Peskov told a conference call with reporters on October 31 that "Syria-related issues will be on the agenda of the Russian-Iranian bilateral talks." He did not elaborate.
Russia and Iran back President Bashar al-Assad's government in Syria's more than six-year war, while the United States and Turkey back differing rebel groups.
Russia is also one of the signatories of the 2015 international accord under which Tehran agreed to curb its nuclear program in exchange for sanctions relief.
U.S. President Donald Trump refused to recertify the nuclear deal in October and has threatened to withdraw the United States if what he calls serious flaws in the accord cannot be fixed by U.S. lawmakers and U.S. allies.
The European Union and the other parties to the accord -- Russia, Britain, China, France, and Germany -- also have all urged Trump not to pull the United States out.
Speaking in Moscow on November 1, Russian Deputy Foreign Minister Sergei Ryabkov said that the deal must be preserved and that new sanctions should not be imposed on Iran. He called Trump's demands for changes "unrealistic."
Russia has also aided Iran's nuclear energy program, making a major contribution to the construction of its first nuclear power plant in the southern city of Bushehr, where Moscow is now helping build two more reactors.
Putin made previous trips to Iran in November 2015 and in 2007.
Ahead of Putin's arrival, Russia's armed forces chief of staff, General Valery Gerasimov, met with his Iranian counterpart, General Mohammad Hossein Bagheri, and discussed defense and security issues, Iranian state television said.
The visit comes a day after representatives from Russia, Iran, and Turkey discussed the Syrian conflict in Astana, Kazakhstan, and pledged to bring Assad's regime and its opponents together for a "congress" to jump-start peace efforts later this month.
A statement on the Russian Foreign Ministry website on October 31 listed 33 Syrian organizations invited to a Congress of Syrian National Dialogue in the Russian city of Sochi on November 18.
There have been seven rounds of Astana talks, which began in January and are sponsored by Russia, Turkey, and Iran.
The Astana talks have run parallel to negotiations taking place in Geneva with the backing of the United Nations, and the congress plan appears to be Moscow's attempt to force the pace in a bid for a political settlement.
But representatives of the Syrian opposition in Astana cast doubt on the plan.
Yehya al-Aridi, a spokesman for the Syrian opposition, called the suggestion "a fake message" on Syria.
With reporting by Reuters, dpa, and Interfax