LONDON – Europe began to emerge from a volcanic cloud Monday, allowing limited air traffic to resume and giving hope to millions of travelers stranded around the world when ash choked the jet age to a halt.

Despite the progress, the eruption from the Icelandic volcano that caused the five days of aviation chaos was said to be strengthening and sending more ash toward Britain, which could make it unlikely that London airports would reopen today.

Three KLM passenger planes left Schiphol airport in Amsterdam on Monday evening under visual flight rules, bound for New York, Dubai and Shanghai. An Associated Press photographer saw one jet taking off into a colorful sunset, which weather officials said was pinker than normal due to the ash.

European Union transport ministers reached a deal Monday during a videoconference to divide northern European skies into three areas: a “no-fly” zone immediately over the ash cloud; a caution zone “with some contamination” where planes can fly subject to engine checks for damage; and an open-skies zone.

The German airline Lufthansa said it would bring 50 planeloads of passengers home today.

But the optimism was tempered Monday night by a statement from the British National Air Traffic Service, which said the eruption of the volcano has strengthened and a new ash cloud was spreading toward Britain.

The service said airspace over some parts of England may be reopened this afternoon but that the open zone for flights may not extend as far south as London, where the country’s main airports are located. It also indicated that Scotland’s airports and airspace can reopen as planned today but said the situation in Northern Ireland was uncertain.

Visual flight rules allow a pilot to fly without reference to instruments, if weather conditions are good enough so the pilot can see landmarks and avoid other aircraft. The flights need to be under 18,000 feet, lower than the usual altitude for commercial traffic.

Scientists have instruments that can both detect the presence of the ash and measure its concentration — information that can be relayed to pilots.

Test flights in recent days by airlines including KLM, Lufthansa and British Airways suggested planes can fly safely despite the ash. None of the flights reported problems or damage.

“The analysis we have done so far, alongside that from other airlines’ trial flights, provides fresh evidence that the current blanket restrictions on airspace are unnecessary,” said BA chief executive Willie Walsh. “We believe airlines are best positioned to assess all available information and determine what, if any, risk exists to aircraft, crew and passengers.”

Scientists and pilots urged caution.

“Mixing commercial and safety decisions risks lives,” said Philip von Schoppenthau, secretary-general of the European Cockpit Association, a union representing 38,200 pilots from 36 nations.

“Our members have many firsthand experiences of the extremely abrasive and clogging effects of such clouds,” he said.

Millions of travelers have been stuck since the volcano under Iceland’s Eyjafjallajokull glacier begun erupting April 14 for the second time in a month, spewing a vast cloud of ash that has drifted over most of northern Europe and is now spreading west toward North America.

White House press secretary Robert Gibbs said about 40,000 Americans in Britain were stranded, citing Louis Susman, the U.S. ambassador to Britain.

“We are working closely with the State Department to examine all the opportunities that we have to speed this process along,” Gibbs said. “. . . They’re running out of medicine. They don’t have a place to stay.”

Eurocontrol, the air traffic agency in Brussels, said less than one-third of flights in Europe were occurring Monday — between 8,000 and 9,000 of the continent’s 28,000 scheduled flights. Officials said more planes would operate today, although it wasn’t immediately clear how many.

German Transport Minister Peter Ramsauer said all planes under the “control zone” plan will be thoroughly checked once they’ve landed.

“Much stricter tests and checks will be applied to all planes,” Ramsauer said, in hopes of gaining more data about the risk from the ash. “Nobody knows how long the situation will continue.”

Airports in central Europe and Scandinavia have reopened, and most of southern Europe remained clear, with Spain volunteering to be a staging point for overseas travelers trying to get home. Infrastructure Minister Jose Blanco said Spain could take in about 100,000 people under the emergency plan, which focuses on trying to bring Britons home from Asia, Latin America and North America.

Tensions boiled over among frustrated passengers at Incheon International Airport in South Korea, where 30 people blocked a Korean Air ticketing counter and demanded officials arrange travel to anywhere in Europe. “We need a flight, we need a time,” Thierry Loison, who has been stuck at the airport since Friday on the way back to France, told Korean Air officials. “We were like animals .”