Pakistan said it had begun applying the new checks on US-bound passengers
Security has been stepped up at many airports in the wake of the alleged bomb plot against a US plane as it approached Detroit on 25 December.
Some countries have announced plans to introduce more sophisticated technology to screen travellers, others are making body and hand-baggage searches more rigorous.
The BBC looks at what changes are being made in a selection of countries.
US Transport Security Administration
announced that from 4 January 2010 anyone flying to the US from or through nations considered state sponsors of terror, as well as "other countries of interest", would have to undergo
additional security checks
The US considers Cuba, Iran, Sudan and Syria to be state sponsors of terrorism, while "countries of interest" include Afghanistan, Algeria, Iraq, Lebanon, Libya, Nigeria, Pakistan, Saudi Arabia, Somalia and Yemen.
The new directives - applied irrespective of passengers' nationality - come on top of new "long-term, sustainable" security measures, including pat-downs and bag searches at the gate, which would apply to all US and international airlines at all airports, it said.
During flights, passengers may be asked to follow flight crew instructions, such as stowing personal items, turning off electronic equipment and remaining seated during certain portions of the flight. Such measures will be applied at the "discretion" of the cabin crew.
UK Prime Minister Gordon Brown announced on 3 January that he had given the go-ahead for
to be introduced at Britain's airports.
BAA, which runs six UK airports, said it would now install the machines "as soon as is practical" at Heathrow, the world's second busiest airport.
Alongside the new scanners, airports are expected to bring in more bomb sniffer dogs and increase their use of passenger profiling. Hand-luggage is also expected to be swabbed for traces of explosive.
In the meantime, extra departure gate checks introduced since Christmas remain in place. These include a pat-down and bag search.
Department of Transport
told the Associated Press that it was still trying to decipher the practical implications of the new US rules.
- where Nigerian Umar Farouk Abdulmutallab transferred onto the flight to Detroit before he
allegedly attempted to detonate a bomb
- security guards are already using two full-body scanners on passengers flying to the US.
The airport is also trying to quickly fit 13 other scanners with special, less invasive software to allay privacy concerns. Until then, security staff are performing full-body searches on all US-bound passengers.
Schiphol already swabs hand-luggage for traces of explosives, but only luggage belonging to passengers who attract special scrutiny.
After the failed bombing, the French authorities ordered security officers to perform more body searches and extra checks on hand baggage, particularly on those flying to the US.
Checkpoints have been set up at departure gates, where all carry-on items are opened and inspected, and travellers are patted down.
Officials in Germany said no new measures were introduced on 4 January, because airport security had been increased after Christmas Day's failed attack.
The government has yet to decide whether to introduce full-body scanning, with many experts and politicians saying it would invade people's privacy.
Security has been tightened at
Rome's Fiumicino airport
Milan's Malpensa airport
, the country's two busiest, since a suspicious package with a detonator was discovered at the former last week.
Italy's civil aviation authorities had already ordered all airports to be vigilant about flights to the US, and conduct more thorough body and baggage searches.
Interior Minister Roberto Maroni said the government wanted full-body scanners to be installed quickly at Fiumicino and Malpensa and used on passengers of flights considered at high risk of attack. He said Italy wanted to overcome concerns about privacy held by many within the European Union.
Though security has been increased at Spanish airports in the past week, one official told the Associated Press on 4 January that US-bound passengers from countries on the new watch list were not being singled out for body searches.
Passengers in Oslo bound for the US faced heavy security on 4 January. They were forced to show their passports and boarding passes twice at the departure gate, had their carry-on baggage searched, and went through full pat-downs.
implemented new security regulations for flights to the US following the 25 December incident. Currently, US-bound passengers are not permitted to take carry-on luggage into the cabin, with the exception of small handags, cameras, coats, medication, laptops and walking-sticks.
The transport ministry said all passengers flying to the US would continue to be patted down and have their carry-on baggage searched. New security measures, including an increase in the number of
Australian Federal Police (AFP)
officers at the country's 11 major airports, were introduced recently.
An official at Seoul's
Incheon International Airport
told the Associated Press that US-bound passengers were now required to go through additional security before boarding, and that more thorough screening had been introduced.
Nigeria's government has said it will introduce full-body scanners as soon as possible and perform whatever security checks the US authorities request.
The new rules led to long queues at security at Lagos'
Murtala Muhammed International Airport
on 4 January, where some travellers were reportedly told to check in more than seven hours ahead of their flight to Atlanta.
There are no direct flights to the US from Pakistan, but
Pakistan International Airlines
said it had begun applying the requested security checks on US-bound passengers, including full-body searches.
A spokesman for Baghdad International Airport noted that it already had security procedures in place that were stricter than at any other airport in the world. Passengers have their luggage sniffed by dogs and are patted down before they are allowed to even enter the airport.
Extra security personnel have been deployed at the country's airports.
OTHER COUNTRIES ON US WATCHLIST
No changes were seen on 4 January at international airports in Syria, Algeria, Libya or Lebanon, according to the Associated Press.