Damascus (AFP) - Syria has denied reports of increased military activity by Russian troops on its soil, after Washington said it was following up on claims of ramped up support from Moscow.

Speaking late Monday to Hezbollah's Al-Manar television station, Information Minister Omran Zohbi dismissed the reports as baseless.

"There is absolutely nothing to these rumours and what was said a few days ago," Zohbi said of reports of increased aid from Russia, a key supporter of Syrian President Bashar al-Assad.

"There are no Russian forces, and there is no Russian military activity on Syrian territory by land, sea or air," Zohbi told Al-Manar, which belongs to powerful Lebanese Shiite militant group Hezbollah, also allied with Assad.

On Thursday, the White House said it was following up on reports that Russia was conducting military operations inside Syria, warning that any confirmed activity would be "destabilising and counter-productive".

US Secretary of State John Kerry had told his Russian counterpart Sergei Lavrov that the US was "concerned" about Russian military reinforcements in Syria, the State Department said.

Moscow said that the aid it provides to Damascus is normal.

Russia maintains a naval base in the Syrian port of Tartus, the origins of which date back to Moscow's close relationship with Damascus under the Soviet Union.

Zohbi said the rumours about increased Russian aid were "circulated by Western intelligence and in some Arab intelligence services to give the impression... that Russia is intervening directly in order to put pressure on Syria... and that the Syrian state has weakened so much it needs direct help from its friends."

"Syrian-Russian ties in the military context are a prolonged relationship, and whatever is coming from the Russian military to Syria is a result of previous agreements that were settled in the past, and are not something new," Zohbi told Al-Manar.

He accused the US and others of providing "lethal aid to armed terrorist groups" that he said were falsely presented as being part of the moderate opposition.

A Greek official said on Monday that his country had received a request from the US to block Russian supply planes heading to Syria from flying through Greek air space.