BRUSSELS (AP) — Taking an act-now-ask-questions-later approach, countries around the world slammed their doors shut again to try to keep the new omicron variant at bay Monday as more cases of the mutant coronavirus emerged and scientists raced to figure out just how dangerous it might be.

Japan announced it would bar entry of all foreign visitors, joining Israel in doing so just days after the variant was identified by researchers in South Africa. Morocco banned all incoming flights. Other countries, including the U.S. and European Union members, have moved to prohibit travelers arriving from southern Africa.

Travelers infected with the new version have turned up in a widening circle of countries over the past few days, including now Spain, and cases in Portugal and Scotland have raised fears that the variant may already be spreading locally.

“Many of us might think we are done with COVID-19. It’s not done with us,” warned Tedros Adhanom Ghebreyesus, director-general of the World Health Organization.

Days after the variant sent a shudder through the financial world nearly two years into the pandemic that has killed over 5 million people, markets had mixed reactions Monday, with European stocks and oil prices rebounding and Wall Street opening higher, while Asian markets fell further.

U.S. President Joe Biden called the omicron variant a cause for concern but “not a cause for panic.” He said he is not considering any widespread U.S. lockdown and instead urged vaccinations and mask-wearing.

The infections have underscored the difficulty in keeping the virus in check in a globalized world of jet travel and open borders. Yet, many countries are trying to do just that, against the urging of the WHO, which noted that border closings often have limited effect and can wreak havoc on lives and livelihoods.

Some argued that such restrictions could provide valuable time to analyze the new variant. Little is known about it, including whether it is more contagious, more likely to cause serious illness or more able to evade vaccines.

While the initial global response to COVID-19 was criticized as slow and haphazard, the reaction to the new variant came quickly.