Attacks spark global red alert

South Korean police officers arrive at the U.S. embassy in Seoul, South Korea

LONDON, England -- The decision to put U.S. military forces worldwide on high alert following Tuesday's attacks has been followed by similar moves by national governments around the globe.

Not only were security forces put on top alert among the United States' NATO allies -- including Britain, France and Germany -- but also in Russia, China, parts of the Middle East and Asian nations.

NATO's Brussels headquarters and the allied forces headquarters at Mons, also in Belgium, have been placed on a state of maximum alert.

Moscow stepped up security across the country and halted all flights to the U.S..

The forces of the country's interior ministry were also put on a state of maximum readiness as president Vladimir Putin said in a televised address that the world should unite to combat the threat of terrorism, "the plague of the 21st century."

European Union foreign ministers said they would hold emergency talks on Wednesday while NATO ambassadors were holding an extraordinary meeting on Tuesday night to discuss the wave of attacks.

European Commission officials were summoned home immediately from any overseas visits.

At the same time, the European Central Bank gave an emergency injection of liquidity into monetary markets, meant to stabilize volatile financial swings.

United Nations aid workers prepared to leave Afghanistan on Wednesday, U.N. sources told CNN.

In addition, smaller aid groups working for non-governmental organisations began to leave their compounds in the capital, including 14 German groups who will move out over the next couple of days.

But the focus of the many security measures introduced across the international community centred on airports and embassies, with many international airlines scrambling to divert or cancel flights to the U.S..

The cancellations and diversions caused confusion and congestion at many European airports, where airlines ordered flights bound for the U.S. to carry out U-turns or find alternate landing points outside America.

Some airlines reversed course only after being denied permission to land by the Federal Aviation Administration, which took the unprecedented step of ordering the complete shut down of U.S. air space.

The move affects 36,000 to 40,000 flights that take off in the U.S. daily, as well as general aviation flights.

"The earliest the national groundstop will be lifted is noon tomorrow. And that's at the very earliest," FAA spokesman Les Dorr told CNN on Tuesday.

Similar air space closures were implemented by Canada, Britain and Belgium, where commercial flights over their capitals of London and Brussels were banned.

A Chinese military policeman stops a cyclist from entering the road leading to the U.S. embassy in Beijing

British Prime Minister Tony Blair said: "No flights will take off from the UK for which we cannot apply the highest standards of security for aircrew and passengers.

"Private flights have been stopped except where specifically authorised."

The British Foreign Office told CNN that it had advised British nationals across the world to stay at home and to avoid crowded places where they could draw attention to themselves.

Thousands of workers and shoppers were evacuated from the world's tallest buildings, the 88-storey Petronas Twin Towers on Wednesday, following a bomb threat that came as Malaysians were just learning of the tragedy gripping the U.S..

Police bomb squads found no explosives in the 88-storey towers and people were allowed back to all floors shortly before noon (0400 GMT), a little over three hours after the threat.

Just as the Petronas towers were declared safe, a threat was phoned in to the IBM building in Kuala Lumpur. Some 2,000 people were evacuated as police combed the 24-storey landmark, which houses the offices of IBM Malaysia, Standard Chartered Bank and other firms.

In Canada, prime minister Jean Chretien said: "Everyone has increased the level of security everywhere ... here in Canada we have increased security adequately.

Chretien said the country's airports had been shut along with all the nation's consulates in the U.S..

In France, armed troops were deployed at airports and metro stations. Border controls have increased.

The U.S. Embassy in Paris was closed and will be shut again on Wednesday. The embassy is warning Americans in France to be careful about speaking English on the streets.

In Germany, the interior ministry said it had set up a cross-ministerial crisis committee, while security was increased at government and U.S. and Israeli installations across the country. It also said flags on all public buildings in Germany would fly at half mast for two days.

The regional government in the state of Hesse recommended the main skyscrapers in Frankfurt, Germany's financial capital and home to the European Central Bank, close on Wednesday.

Despite that, Frankfurt's equivalent to the World Trade Center, the Messeturm building, was open for a short while only to be evacuated early on Wednesday after police received a bomb alert.

Security officers cleared the building at 9 a.m. (0700 GMT) following the anonymous call which said the device would explode at 9.20 a.m.. The all-clear was given 15 minutes later.

About 4,000 employees work in the building, mainly for investment banks.

The ECB said it would take a decision on Wednesday on whether staff should return to its headquarters.

Closings, cancellations

The new Jewish museum in Berlin cancelled its public opening set for Tuesday night.

The U.S. embassy in Rome sent staff home early, while Spain threw cordons around the U.S. and Israeli embassies and the Palestinian representation.

Romanian special forces cordoned off the U.S. embassy in Bucharest, which shut down for two days last week after unspecified threats to security, as well as the Israeli embassy.

Greece, criticised by Washington over its anti-terrorism record, ordered a security alert at the U.S. embassy in Athens and other potential U.S. targets, including schools.

Spain has stepped up security at its airports, where normal operations were reported on Wednesday. U.S.-bound flights from Spain were returned late on Tuesday or a few were diverted to other airports.

Israel closed its air space to foreign planes, closed land crossings and evacuated staff from diplomatic missions and Jewish institutions around the world.

World Bank President James Wolfensohn told CNN it will be some days before he makes a decision about canceling or postponing the IMF/World Bank annual meetings, scheduled for September 29-30.

He said dealing with the tragedy was where the focus should be for now.