The US has said international flights to or over the country will be required to carry armed guards in certain cases.
The US measures take immediate effect
The US Secretary for Homeland Security, Tom Ridge said flights could be banned if airlines refused to comply.
The US decision comes amid renewed fears that terrorists may be trying to use aircraft to stage a new attack.
The UK has already announced it will put undercover armed police on some flights - a move opposed by many pilots and some airlines.
Germany's Lufthansa airline began carrying sky marshals on some flights to the US after the 11 September attacks, a spokesman said.
The US directive, which has come into immediate effect, applies to passenger and cargo flights, even if they are just passing through American airspace.
"This is another in a long list of measures that we have taken in the last two and a half years to increase aircraft security," US Homeland Security Secretary Tom Ridge said at a news conference.
"This is an international challenge that we all have."
According to the regulation, "where necessary" foreign carriers "will now be required to place armed, trained law enforcement officers on designated flights as an added protective measure".
Department spokesman Dennis Murphy said they would call for sky marshals depending on the intelligence they received.
"We will notify airlines when we have information on a specific flight," he said.
Just before Christmas, Mr Ridge announced that the state of alert in the US was being stepped up in response to a substantial increase in intelligence reports about possible threats.
Airline security was increased, amid reports that extremists were interested in using aircraft as in the 11 September 2001 attacks.
Last Wednesday and Thursday, six Air France flights between Paris and Los Angeles were cancelled amid fears of a terror attack.
US intelligence has suggested that al-Qaeda members posing a "credible threat" were due to board.
The Australian Government announced on 26 December that armed guards would be deployed on many flights between Australia and Singapore.
Sky marshals armed with live ammunition and stun guns have been operating on some Singapore Airlines flights since August.
Armed marshals disguised as passengers are already deployed on thousands of US flights each week.
Captain Dennis Breslin, an American Airlines pilot, said he would not fly without their protection.
"Because between the door of the cockpit and the cabin, if the door is breached and those pilots are taken out as they were on 9/11, you're all doomed," he told BBC News.
But the British airline pilots' union has expressed strong opposition to the use of air marshals, saying it made flying more dangerous.
And the International Air Transport Association (IATA), which represents around 95% of airlines operating international flights, is also opposed.
"IATA does not like guns on board planes. We want to see the security measure taken on the ground before anyone gets as far as the plane," spokeswoman Nancy Gautier told Reuters.