Closing borders would be "disproportionate and ineffective" against COVID-19, European ministers have agreed — despite several countries announcing their first cases of the coronavirus.

A coordinated response to travellers returning from risk areas was agreed on Tuesday by health ministers from countries including France, Germany and Italy.

They also agreed there was no need to cancel major public events, but to "evaluate case by case the appropriate measures to be taken" to reduce the spread of the disease.

It came after the first cases of COVID-19 were confirmed in mainland Spain as well as Switzerland, Austria and Croatia.

Clusters of the illness continued to balloon outside mainland China, fueling worldwide worries that were reflected in weak financial markets.

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On the Spanish holiday island of Tenerife, 1,000 tourists were being prevented from leaving a hotel where an Italian man had symptoms.

In Cheshire, England, a school closed after pupils returned from a ski trip to northern Italy.

Austria said its first two confirmed cases were being isolated at an Innsbruck hospital, and that a hotel near the city was in quarantine.

The Croatian case is a man who had travelled to Turin.

In Switzerland, authorities confirmed a case in the canton of Tessin, close to the border with Italy.

Iran's official death toll climbed to 15, with 95 confirmed infections — including the head of the country's coronavirus task force.

Italy is trying to deal with the biggest cluster of infections in any country outside of Asia, with 322 cases including 11 deaths.

It comes after the World Health Organisation (WHO) on Monday urged countries to act "to prevent a potential pandemic."

The EU has pledged €230 million to fight the outbreak in Italy and elsewhere but said it would not yet impose restrictions on travel or trade.

WHO says hidden iceberg of mild cases unlikely

Bruce Aylward, head of WHO mission to China and who recently returned from Wuhan, played down reports that a large number of COVID-19 carriers with mild symptoms could be spreading the virus without knowing they were infected.

"There is not huge transmission beyond what you can actally see clinically," he told reporters in Geneva. "All the data that we have suggests there isn't this massive iceberg."

Asked about online theories that the true death toll in China is in the millions, he said: "I didn’t go to every single place, every corner of China, but we have a pretty good sense of what this epidemic looks like and [our] numbers are reflective of that."

He also urged developed countries to be prepared as if the virus was already in the country. "It will show up," he said, adding that the hardest task for governments is persuading citizens to accept quarantine and isolation.

Italy grapples with the spread of COVID-19

Italy has locked down at least ten towns in the northern state of Lombardy and the epicentre of the Veneto cluster, Vo'Euganeo.

Police are manning checkpoints around quarantined towns, and police also stepped in to regulate customer queues at some supermarkets, as people stockpiled food in the wake of the lockdown order.

Sicily reported a new coronavirus case on Tuesday — a woman on holiday in the north. She and her husband were put in isolation.

Italy's Civil Protection chief Angelo Borrelli urged Italians to abide by the containment measures for the two week quarantine period.

"If in a certain area at first there was a recommendation and then an obligation to stay put, we are respecting that and it will be good for us and good for others," said Borrelli.

Many of the new cases represented the first infections in Italy acquired through secondary contagion.

Some of the cases came at the same hospital in Codogno, one of the Lombardy towns now on lockdown.

The mayor of Codogno issued a decree ordering the closure of all restaurants, coffee bars, schools and public gathering spots such as discos and gyms. The health ministry advised area residents to stay home as a precaution.

Italian Prime Minister Giuseppe Conte called for a meeting Tuesday of all Italian government ministers and regional governors to come up with a coordinated plan for coping with the spread of the new coronavirus across the country.

He acknowledged that regions have been handling the emergency situation with different measures creating "confusion" and noted that on Tuesday they hope to come up with a coordinated protocol for the country.