People around the globe are being warned by their governments of the danger of traveling to Brussels in the wake of the deadly terrorist bomb blasts that killed at least 34 people. The US is alerting citizens to take extreme caution not only in Belgium but throughout Europe, and the Netherlands is telling its residents to avoid Brussels altogether.

The British government is warning its citizens to be cautious when visiting Belgium, which has now been placed at threat level 4 — meaning "a serious and imminent threat." "You should remain alert and vigilant, especially in places where there's a high concentration of people," said the UK Home Office.

The US State Department issued an alert to American citizens traveling to and throughout all of Europe. "Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation," said the alert issued on 22 March .

"Be aware of immediate surroundings and avoid crowded places. Exercise particular caution during religious holidays and at large festivals or events."

The Australian government has urged citizens to use a "high degree of caution" when traveling to Brussels, and the Netherlands is telling its residents to avoid travel to Brussels.

"Those who are in Brussels already are advised to stay indoors wherever possible," said a statement on the Dutch Foreign Ministry. "The city's metro stations and airport should also be avoided."

Traveling to and within Brussels in any case is currently difficult, with its transportation system essentially on lockdown. The airport will be closed at least through Wednesday 23 March. "Brussels Airport has been shot in the heart," said Airport CEO Arnaud Feist.

The entire Brussels public transport system was closed on the day of the blasts, but select metro, tram and bus lines were starting to resume service on 23 March, according to the system's website.

Eurostar is resuming regular service. Paris' Gare du Nord station — where Eurostar trains leave for London — was evacuated hours after the terrorist attacks in Brussels following the discovery of an abandoned suitcase at the station. But it turned out to be a false alarm.

Security has been tightened at European and American airports and in US cities — particularly in New York, Washington DC and Los Angles, reports CNN.