Russia's weather and emergency officials soothed fears of Moscow residents Thursday with statements that green-tinged clouds over the capital were not an alien invasion, but tree pollen.

"Today Muscovites felt like characters in a disaster film about an alien invasion: people living in the southwest of the city saw that the sky had been coloured green," said Russia's weather service on its website.

The clouds crept up on the Russian capital from the south in the morning, and reached the centre by the afternoon, causing office workers to gawk at the suspiciously colored sky.

"Green clouds are coming toward Moscow," Komsomolskaya Pravda tabloid headlined its story. "Could it be that the apocalypse is upon us?"

Green dust also covered streets and cars. Some people in Moscow and the region apparently called emergency numbers in a panic, leading officials to say the air was thick with tree pollen, not disaster fallout.

"Many residents forgot all about natural phenomena and decided that it's the result of an accident at an industrial facility," said the city administration of Moscow region town of Podolsk, an industrial town south of Moscow.

"But this dust is pollen from alder and birch trees, which began flowering recently after a slow spring," it said on its website.

The emergency situation ministry said the sudden onset of spring and rapidly rising temperatures "caused blooming of several species of trees, and resuted in a yellow-green pollen coating over pavement, windows, and cars."

"The pollen poses danger to people suffering from allergies and asthma, for others it's only seasonal discomfort," the ministry said.