Police evacuated Grindavik in November after strong seismic activity in the area damaged homes and raised fears of an imminent eruption. Officials said magma could continue in that direction.
Mexico has deployed some 7,000 soldiers to the area of the volcano, which is near Mexico City, in case an evacuation becomes necessary. More than 25 million people live within 60 miles of the peak.
Two wharf workers tested positive Tuesday. The Ministry of Health has been testing workers there amid international aid shipments after last month's volcanic eruption.
A station to detect nuclear weapons tests picked up the volcanic eruption in Tonga from Antarctica. Some experts say the blast could be more than 50 megatons, while NASA estimates 6-10 megatons.
Authorities on the Canary Islands declared an eruption that started in September officially finished following 10 days of no lava flows, seismic activity or significant sulfur dioxide emissions.