Tens of thousands of passengers face fresh travel chaos because of stronger eruptions from the volcano, which already closed Heathrow during the first ash alert last month.

Gatwick Airport in the British capital was also among those shutting for six hours Monday, said the National Air Traffic Services (NATS), which manages British airspace.

"The high density ash cloud continues to move further south in the early hours" of Monday, said a NATS statement.

The new closures applied "for the period 1:00 am (0000 GMT) until 7:00 am (0600 GMT) Monday," said the air traffic controllers.

Europe's skies were partially closed for up to a week in April following the eruption of Iceland's Eyjafjoell volcano, in the biggest shutdown of the continent's airspace for more than 50 years.

Experts fear that the volcanic ash can cause enough damage to jet engines to create a serious risk of a crash.

A University of Iceland vulcanologist said Eyjafjoell activity had recently risen.

"There is slightly increased activity for the past two days, there has been some ash fall around the glacier," said Bjoern Oddsson.

"The column (of smoke) has increased and rises up to eight kilometres (five miles)," he told AFP, as opposed to six kilometres in previous days.

Its effect on European flights "all depends on the winds", said the geologist.

In their latest update, NATS also ordered the closure of several smaller airports around London and southeast England early Monday and said the whole of Northern Ireland remained inside the "no-fly zone" in the same period.

Dublin Airport was to remain closed until 1100 GMT, after Irish regulators ordered it to shut down on Sunday. Several other airports in the republic would remain shut for at least several hours early Monday.

There was relief for travellers using airports in northern and central England, however, with aviation chiefs saying they would likely reopen from 0000 GMT as the ash cloud drifted south and away from their airspace.

Manchester Airport in northwest England, Britain's busiest outside London and among the 20 busiest in Europe, was among those likely to reopen after being closed for several hours Sunday, said NATS.

Others set to start receiving flights again included Birmingham, Liverpool, and Leeds-Bradford.

In Scotland, Prestwick was to reopen but Aberdeen Airport, in the northeast, was set to close and some island airports were still shut.

After new closures started to be brought in Sunday, Eurostar -- which runs high-speed rail services linking London with Paris and Brussels via the Channel Tunnel -- said it was laying on extra trains.

More services would run between the British and French capitals Monday to answer an expected surge in demand, said the train company.

Earlier, aviation regulators in the Republic of Ireland extended a closure of Dublin airport by three hours to at least 1100 GMT Monday. The airport closed at 1800 GMT Sunday.

The other two main airports, Cork and Shannon, remain open until further notice, the Irish Aviation Authority (IAA) said.

Of the smaller hubs, Donegal was to remain closed until at least 1100 GMT, while Ireland West (Knock) and Sligo were to reopen at 0800 GMT.

Kerry is open until further notice, while Galway and Waterford would reopen at 0500 GMT Monday.

North Atlantic flights crossing Irish airspace would not be affected, said the IAA.

In Scandinavia, the skies were open over Denmark, Finland, Norway and Sweden and should remain so until at least Monday night, when there is a chance of Danish airspace being affected.

There were no restrictions on German flights, with the DFS air safety authority saying that ash pollution was "weak" for the moment.

rjm-burs/sr/jj

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