Volcanic Italian Island on Alert
The last major eruption in 2002 caused a collapse that led to a small tsunami.
The wave reached 10 metres (33ft) high and caused serious damage to Stromboli village on the north of the island.
'Clearing zones'
The BBC's Christian Fraser in Rome says the new lava flow is heading down the west flank of the volcano and at the moment poses little risk. But experts say it has to be watched closely.
An eruption on Stromboli in 2002 caused a small tsunami (Photo at right)
A member of the civil defence authorities for the Aeolian Islands told Reuters news agency: "There's a general alert. The emergency services are monitoring the situation, clearing zones at risk.
"But at the moment we don't foresee an evacuation."
The volcano, which rises 2,000 metres from the ocean floor, has been active for the past 2,000 years.
Most eruptions consist of gas explosions that hurl small blobs of lava over the crater rim. There are several explosions each hour.
The larger eruptions and lava flows are less frequent.
The one in December 2002 forced the closure of the island to visitors for a number of months.