Russian President Vladimir Putin began a visit to Iran on Wednesday designed to underpin closer ties between two countries at loggerheads with the United States as President Donald Trump threatens to pull out of a 2015 nuclear deal with Tehran.Putin and his Iranian hosts are expected to discuss the nuclear deal and regional crises such as the Syrian conflict, in which Moscow and Tehran are the main backers of President Bashar Assad, while Washington, Turkey and most Arab states support opposition groups seeking to overthrow him."We are very pleased that, apart from our bilateral relations, our two countries play an important role in securing peace and stability in the region," Iranian President Hassan Rouhani told Putin in his welcoming remarks.Russian and Iranian help has proved crucial for Assad, allowing him to win a series of military victories since 2015 and to reestablish his control over most of Syria. Moscow is now trying to build on that success with a new diplomatic push, including a meeting in the Black Sea resort of Sochi next month.Moscow is also an important ally for Iran in its confrontation with the Trump administration, which on October 13 refused to certify Tehran's two-year-old nuclear deal with six major powers that include Russia and the United States.Russia has criticized Trump’s move, which has opened a 60-day window for Congress to act to reimpose economic sanctions on Iran. These were lifted under the 2015 accord in return for Tehran curbing its nuclear program."This is a very important visit (by Putin) ... It shows the determination of Tehran and Moscow to deepen their strategic alliance.... which will shape the future of the Middle East," one Iranian official told Reuters, on condition of anonymity."Both Russia and Iran are under American pressure ... Tehran has no other choice but to rely on Moscow to ease the US pressure," said the official.Another Iranian official said Trump's aggressive Iran policy had united Iran's faction-ridden leadership in alignment with Russia.During his visit, Putin will also discuss boosting bilateral economic ties, and will take part in a three-way summit between Russia, Iran and neighboring Azerbaijan, state TV said.