Cuban paramilitary and special forces units have been spotted on Syrian grounds possibly to aid President Bashar al-Assad and assist Russia in fighting the rebel forces, according to US intelligence reports.

Citing an official who spoke under the condition of anonymity, Fox News said two Russian planes landed at Damascus airport and unloaded Cuban troops. The troops were said to man Russian-made tanks to aid Damascus in fighting rebel forces backed by the US.

The University of Miami's Institute for Cuban and Cuban-American Studies also issued an unconfirmed report that Gen. Leopoldo Cintra Frias, head of Cuba's armed forces, was seen in Syria leading a group of Cuban military personnel joining forces with Russia in their support of Assad.

"When the troops were asked by an Arab military officer at the Damascus airport they told him they were there to assist Assad and that they are experts at operating Russian tanks,'' Jaime Suchlicki, the institute's executive director, said.

"It doesn't surprise me... they have a very close relationship," Suchlicki told Fox News, citing Russia's long history of supplying military equipment to Cuba as well as Cuba's assistance in Soviet-led operations in Africa the 1970's.

"The Russians have been training the Cubans for years and supplying them with all sorts of military equipment,'' he added.

Last week, the Cuban government categorically denied the reports that its troops have been sent to Syria, calling it "irresponsible and unfounded.''

The war in Syria, which is now entering its fourth year, has so far displaced an estimated 250,000 people resulting in a humanitarian crisis. The US and its coalition allies continue to insist that Assad must go while supporting a rebel group known as the Free Syrian Army in the fight against the Islamic State militants, which have been forming a caliphate across Syria and Iraq.

Russian warplanes, Iranian troops and the Lebanese militia Hezbollah fighters meanwhile continue their ground offensive in areas seized by the ISIS to ensure that Assad remains in power, the Wall Street Journal said.

The unnamed US official has described Cuban's apparent involvement in Syria as similar to the "Cuba-Angola arrangement" wherein its troops were sent to central African countries in the 1970s to operate on behalf of the Soviets. In 1973, Cuba also reportedly deployed troops to Syria to support them in the Yom Kippur War against Israel and deployed officers to observe Israeli military tactics.

"If this information about the presence of Cuban troops in Syria now is confirmed, it would indicate that General Raul Castro is more interested in supporting his allies, Russia and Syria, than in continuing to normalise relations with the US," the institute said in a statement Tuesday.

The US has this year removed Cuba from the list of state sponsors of terrorism in a bid to normalise relations between the two countries.

Cuba's well-trained military is ranked the world's 110th most powerful by the site globalfirepower.com.