According to the Icelandic Meteorological Office, the eruption of the Grimsvotn volcano "paused" at 2:40 a.m. on May 25 (10:40 p.m. may 24 ET), meaning it was not erupting at the moment. The agency warned, however, that it could begin again, and that only after three months with no eruptions will the volcano be considered dormant.

Meanwhile, some German airports were also reopened on May 25 after cancellations due to the cloud of volcanic ash.

Bremen has reopened and Hamburg is allowing takeoffs and landings, according to Deutsche Flugsicherung, the German aviation safety agency.

Airports in Berlin will resume flights at 2 p.m. local time (8 a.m. ET). The closures affected at least 600 flights in Germany, authorities said.

Poland could be affected later in the day, the European air traffic control agency Eurocontrol said. A portion of the ash from eruption in Iceland spread over Britain by May 24 afternoon, with the cloud reaching London's Heathrow airport around lunchtime, a computer model indicated.

Eurocontrol reported about 500 flights in British airspace were canceled on May 24. Heathrow alone normally operates about 1,300 flights a day, and Europe as a whole has about 29,000 daily.

The ash can be a serious hazard to aircraft, reducing visibility, damaging flight controls and ultimately causing jet engines to fail.