Jaish al-Islam, one of Syria's largest rebel groups, has declared war on Russian soldiers fighting alongside Syrian pro-government forces in the country's civil war, a group spokesman told Middle East Eye on Wednesday.

The spokeman's confirmation comes as Russia is actively building up its military support in Syria, including sending 28 operational aircraft to the country, according to US officials.

Fresh satellite images released on Tuesday appear to show Russia developing two new airbases near the key port city of Latakia, a stronghold for Syrian President Bashar al-Assad.

Jaish al-Islam, thought to be funded by Saudi Arabia, posted a video last Friday showing fighters from the group attacking Bassel International Airport, about 20km from Latakia.

In the video, the fighters say the airport has become a base for the Russian army, and then they shoot off several rockets which they claim targeted a Russian cargo aircraft, although there is no evidence of this in the video.

The Jaish al-Islam spokesman was careful, while Skyping with MEE, to make clear that the group had declared war on Russian soldiers, "not on Russia as a country," he wrote.

In the past week, rebel fighters told Reuters that they have encountered stronger resistance from pro-government forces, particularly in Syria's coastal areas, and that a Russian intervention will prolong the war and encourage foreign backers of the rebels to increase military aid.