BEIRUT - Russian forces are working to expand the tarmac of a major airport in Syria's coastal province of Latakia, a stronghold of President Bashar Assad and his minority sect, a prominent Syrian monitoring group said Sunday.

The report comes amid rising concern among U.S. officials of increased Russian military activity in Syria. President Barack Obama cast the buildup as an effort to prop up the embattled leader, warning Moscow against doubling down on Assad.

Russia, a longtime backer of Syria's government, denies it's trying to bolster Assad and instead says its increased military activity is part of the international effort to defeat the Islamic State of Iraq and Syria (ISIS,) the militant group that has wreaked havoc in both Syria and Iraq.

CBS News senior national security analyst Juan Zarate the latest movement of Russian forces into the war-torn region is pressuring the United States to take notice.

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Last week, Russia set up an air traffic control tower near Syria's Mediterranean port of Latakia -- a sign, Zarate said, that Putin and his administration "want to remain a player there."

"They have an interest in not just the Assad regime, a longtime ally, but their base -- their naval base, which gives them an entrée to the Mediterranean," he told CBS News homeland security correspondent Jeff Pegues.

"They're trying to build leverage as they realize that the United States and the rest of the world is going to try to come to some sort of political accommodation in Syria," Zarate said.

The Associated Press reported earlier this week, quoting a former Lebanese general with knowledge of the Syrian military, that there were plans to build a military base in the coastal town of Jableh, more than 20 kilometers south of Latakia city. It is where the airport currently under development is located.

The airport, known by its old name Hemeimeem, already houses a military base, and has come under shelling from militants who have advanced in the countryside of the province.

It is the second most important government controlled airport in Syria after Damascus airport. The Hemeimeem airport was renamed the Basel al-Assad International airport, after the brother of the current president, who died in a car accident in 1994. According to the Syrian Civil Aviation Authority website, the airport's tarmac is currently 3,000 yards long and 49 yards wide. It only has one terminal, according to the site.

The head of the Britain-based Syrian Observatory for Human Rights, Rami Abdurrahman, said witnesses, including officials inside the airport, describe the tarmac development as planned to ensure larger planes can land in the airport.

"It could mean there will be more supplies or that they want to turn it into an international airport," Abdurrahman said.

The witnesses told the Observatory no Syrian military or civilian officials are allowed near the tarmac. The Observatory relies on a network of activists and witnesses on the ground to report on the Syrian civil war, now in its fifth year. More than 250,000 people were killed in the violence, and nearly half of Syria's pre-war population of 23 million has been displaced from their homes. About 4 million of them are refugees in neighboring countries.

Abdurrahman said there are other plans to develop another rural airport east of Latakia city.

The Alawite sect, from which Assad and his family descend, makes up about 13 percent of Syria's pre-war population. It has historically been centered in towns and villages of Syria's mountainous coast that make up the provinces of Latakia and Tartous. If the regime falls, that heartland could become a refuge for the community - and even for Assad himself - from which to fight for survival against a Sunni rebel insurgency and majority population that has long resented their domination.

Abdurrahman said Russian planes arrived in recent weeks carrying military equipment and hundreds of Russian military advisers. He said the experts are also believed to be studying the expansion of the Damascus international airport.