It is now four months since Russia launched its military intervention in Syria, with air strikes and a build - up of troops on the ground. We last visited Russia's Hmeimim air base in Syria in November 2015, the BBC's Steve Rosenberg says. A lot has happened since. Ten days after our visit, Turkey shot down a Russian bomber near the Turkish-Syrian border, resulting in Russia's first combat deaths of this operation. The skies over Syria are even more crowded now: Britain began conducting its own airstrikes in Syria in December, though their frequency is far lower than Russia's air force. Now, the Russian defence ministry has agreed to take another group of journalists back to Syria. I'm on this four-day trip and I'm intrigued to see what has changed. In an Ilyushin Il-62 jet, we fly from a military airfield on the edge of Moscow to Hmeimim airbase near Latakia. When Moscow launched its air campaign in Syria, it made it clear this would be a temporary operation. But nearly four months on, there is no sign of Russia winding down offensive operations. During our day at Hmeimim, we see a steady stream of Russian bombers taking off and landing. Since our last trip here, the Russians now have use of an additional runway. Behind the runway I spot the S-400 surface to air missile complex. This was installed after Turkey shot down a Russian jet last November on the Turkish-Syrian border. We are shown ammunition being loaded onto bombers. Western governments have accused Russia of causing hundreds of civilian deaths with unguided munitions. I ask defence ministry spokesman Maj Gen Igor Konashenkov to respond to the criticism. "There are no facts and no proof," he tells me. Igor Konashenkov is the only military man authorised to give interviews here on the base. We are told it's not possible to interview Russian soldiers who are serving here. In another part of the base, the Russian military shows us humanitarian aid being loaded onto a transport plane. This will be dropped by parachute to communities around Deir al-Zour in eastern Syria, a town besieged by the so-called Islamic State group.