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.

Even then, however, 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 during daylight under visual flight rules bound for New York, Dubai and Shanghai.

European Union transport ministers reached a deal during a crisis 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.

Starting this morning, "we should see progressively more planes start to fly," said EU Transport Commissioner Siim Kallas. The German airline Lufthansa said it would bring 50 planeloads of passengers home.

But optimism was tempered Monday night by a statement from the British National Air Traffic Service, which said the eruption 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 this morning but said the situation in Northern Ireland was uncertain.

Europe's aviation industry -- facing losses of more than $1 billion -- criticized official handling of the disruption that grounded thousands of flights to and from the continent.

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 any other aircraft. Those flights need to be under 18,000 feet.

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

The airlines said 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," BA chief executive Willie Walsh said. "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 European nations.