Ireland ordered a new flight ban for six hours Tuesday as ash from an Icelandic volcano drifted towards it, while a small section of British airspace was closed, air authorities said.

The new alerts should not disrupt aircraft overflying Ireland from Britain or Europe, or southern British airports including Heathrow, Europe's busiest air hub, authorities in the two countries said.

Airspace across Europe was closed down for up to a week last month after the eruption of Iceland's Eyjafjallajokull, but was re-opened after emergency talks between European governments, airlines and regulators.

The Irish Aviation Authority (IAA) said Monday all flights into and out of Ireland would be grounded from 0600 GMT to midday Tuesday due to the dangers posed by the new volcano cloud.

"Ireland falls within the predicted area of ash concentrations that exceed acceptable engine manufacturer tolerance levels," said the authority in a statement.

"The decision is based on the safety risks to crews and passengers as a result of the drift south of the volcanic ash cloud caused by the north easterly winds."

It added that "over-flights of Ireland from the UK and Europe will not be impacted tomorrow. Flights in mainland Europe will operate normally."

Information from the Volcanic Ash Advice Centre (VAAC) suggested that the no fly-zone' would affect Dublin and other airports across the country, said the IAA.

Hundreds of flights were due to depart and fly into Dublin airport throughout the day, with more from Shannon and Cork in the south of the country and Ireland's smaller regional airports.

Airlines Ryanair and Aer Lingus have warned passengers they face disruptions, with delays and possible cancellations.

The IAA advised all passengers intending to travel to check each airline's website.

Speaking shortly before the new ban was imposed, IAA chief executive Eamon Brennan said winds have already pushed volcanic ash down over Ireland.

"Some of the denser volcanic ash, that's the no-fly zone, is over the (County) Donegal area (in the northwest) and we are concerned about the northeasterly winds moving this down over the rest of the country," he told RTE state radio.

"At the moment we have a slither of denser ash over the midlands," he said.

Meanwhile Britain's Civil Aviation Authority (CAA) warned earlier airlines that "increased concentrations of volcanic ash in the atmosphere are forecast to cause limited airspace closures in Scotland" on Monday evening and Tuesday.

It said that from 1700 GMT Monday airspace over the Outer Hebrides would "be closed to all operations following Met Office advice that concentrations of ash in the atmosphere are expected to exceed the safe levels agreed by manufacturers."

Operations from the islands of Barra and Benbecula would cease because of the aerial shutdown, said the CAA.

The international airline industry body, IATA, said last month's shutdown cost carriers some 1.7 billion dollars (1.3 billion euros) and called on governments to pick up at least part of the cost, angered by their handling of the crisis.

Eurocontrol, the continent's air traffic control coordinator, said more than 100,000 flights to, from and within Europe had been cancelled between April 15 and 21, preventing an estimated 10 million passengers from travelling.