Russian bombers flying from an Iranian air base struck rebel targets across Syria today, Russian and Iranian officials said.
The move dramatically underscores the two countries' growing military ties and highlights Russia's ambitions for greater influence in a turbulent Middle East.
The long-range Tu-22 bombers took off from a base near Hamadan in western Iran and launched raids in Syria's Aleppo, Deir al-Zour and Idlib provinces, the Russian Defence Ministry said today.
Both countries are staunch allies of Syrian President Bashar al-Assad, and the flights marked the first time Russia has launched strikes from Iranian territory.
Iran has long banned foreign militaries from establishing bases on its soil. But the raids today appeared to signal a budding alliance that would expand Russia's military footprint in the region.
Iran and Russia "enjoy strategic cooperation in the fight against terrorism in Syria, and share their facilities and capacities to this end," Iran's National Security Council chief, Ali Samkhani, said, according to the state-run Islamic Republic News Agency (IRNA).
Shia-led Iran has sent thousands of troops and fighters, including members of its Revolutionary Guard Corps, to Syria to bolster Assad - who is from the minority Alawite sect - against largely Sunni rebels. For Tehran, losing a longtime ally to a majority-Sunni uprising would undermine its own influence in the region.
Iranian proxies such as Lebanon's Hizbollah and an array of Shia Iraqi militias have also fought for the Syrian regime. And last year, Russia intervened as well, committing tanks, artillery and combat aircraft to the fight. It also built a new air base in Latakia province in the Alawite heartland.
But until now, Russia's long-range bombers, which require longer airstrips, had to be launched from Russian territory more than 1930km away. Now, those same bombers need to fly only about 640km from Iran to Syria, Iran's Fars News Agency reported. The shorter distance will allow Russia to intensify its air campaign against rebel-held areas.
Both government troops and opposition fighters are now locked in a battle for the strategic Syrian city of Aleppo, where residents face a growing humanitarian crisis. Russia has carried out strikes in support of government troops there, activists say.
Russia's Defence Ministry said that its long-range bombers struck targets linked to Isis (Islamic State) and Jabhat Fatah al-Sham, a group that formally split from al-Qaeda last month and changed its name from Jabhat al-Nusra. The strikes destroyed five major ammunition depots, training camps and three command posts, the ministry said.
But rights groups have criticised both Russia and the Syrian regime for repeated strikes on civilian targets, including homes, schools and hospitals. Russian and Syrian officials have denied those reports.
The New York-based Human Rights Watch said Syrian and Russian troops have used banned incendiary weapons in civilian areas.
"These weapons inflict horrible injuries and excruciating pain," Steve Goose, arms director at HRW, said. "The disgraceful incendiary weapon attacks in Syria show an abject failure to adhere to international law."
Russia's clout, however, will probably continue to rise in the region, analysts said. Its military presence in Iran bolsters Russia's growing image as a central player in the Middle East, challenging American supremacy.
Iran itself is deeply involved in conflicts in Yemen and Iraq, where it holds particular influence.
Iran was quick to provide military supplies to the Shia-led government in Baghdad as Isis militants made their land grab in the summer of 2014, pushing towards the capital.
In recent years, Major General Qasem Soleimani, commander of the Revolutionary Guard Corps' elite Quds Force, has made regular appearances on the battlefield in both Iraq and Syria, becoming the public face of Iran's growing military power.
Moscow's alliance with Iran might also allow Russia to make further inroads into both countries.
Last year, Russia and Iran signed a military cooperation deal focused on training and fighting terrorism. In May, Russia made the long-delayed delivery of the advanced S-300 air-defence missile system, which was ordered by Iran in 2007 amid controversy over its nuclear programme.
With the signing of a nuclear deal last year between Iran and six world powers, the sale went forward, and ties between Russia and Iran have improved markedly.
On Monday, Russian President Vladimir Putin's top Middle East envoy arrived in Tehran to discuss bilateral relations. Russia has also requested the use of Iranian airspace to fire cruise missiles at rebel targets in Syria.