Russian forces are working to extend an airstrip near a military airport in Latakia province which is a stronghold of Syrian Regime leader Bashar al Assad and his minority Alawite sect, the Britain-based Syrian Observatory for Human Rights said on Sunday.
The Observatory said, "Russian forces are building a long runway capable of accommodating large aircraft near the Hemeimeem military airport in Latakia province," adding many Russian workers and military advisers had been brought to the region.
According to the report the Russians were "preventing Syrians, whether military or civilian, from entering the area where they are building the runway."
The Hemeimeem airport is both a military base and the second most important government-controlled airport after Damascus airport.
The director of the Observatory which bases its reports on a network of civilian, military and medical sources inside Syria, said sources had also cited that Russia was extending the Hamadiyeh airport which is another regime stronghold.
On Friday, Cypriot officials said Russia had issued an alert for Cyprus to divert aircraft next week because it is planning military exercises off the Syrian coast.
And on Saturday, Syrian state media reported the arrival of two Russian planes at the airport in Latakia, saying they were carrying humanitarian aid.
Syrian state media sporadically reported the arrival of Russian humanitarian aid, but the flights were the first mentioned by official news organisations since the claims of a military build-up.
Moscow - which has openly admitted its support for the Syrian government - has frequently been criticised by US President Barack Obama for backing Assad.
However, it rejects any claims of a military build-up in the country saying it is just taking part in international efforts to fight against ISIS, which has made large territorial gains in both Syria and Iraq.
Russia to continue support
Russian Minister of foreign affairs Sergei Lavrov announced his country would continue to help the Assad regime in "its efforts to fight terrorism."
Lavrov also criticised the West for rejecting cooperation with Syrian regime forces in the fight against ISIS.
The civil war in Syria started with anti-government protests in 2011 and has so far caused 250.000 deaths.
The government has lost large swathes of territory to rebels including militant groups such as Al Qaeda’s Syrian affiliate Nusra Front and ISIS.
Washington says Russian aid to Assad is responsible for a lack of negotiations between rebels and the regime.