A new order by the US Government for foreign airlines to use armed guards if intelligence suggests a terror threat has drawn a mixed response.
The US measure has taken immediate effect
The Thai prime minister said they did not need sky marshals because there were always "proper checks" on flights departing from Thailand.
Some pilots' associations have voiced grave doubts but several countries welcomed the move.
Flights to or over the US may be banned if airlines do not comply.
The American decision comes amid renewed fears that terrorists may try to use aircraft to stage a new attack.
Thai Prime Minister Thaksin Shinawatra told reporters that armed guards would not be placed on US-bound flights by the country's national carrier, Thai Airways, Reuters reported.
"We don't need to go that far since we always conduct proper checks from our end and our planes stop at one destination," he said.
Concerns over safety and cost have also surfaced.
The British airline pilots' union says it believes armed guards would make flying more dangerous - a view echoed by other pilots' groups.
"This is a very bad idea, because it risks having catastrophic consequences for passengers and crew if there is an armed fight on board between a hijacker and an agent," Mogens Holgaard, head of the Danish Pilots Association, told the AFP news agency.
The International Air Transport Association, which represents around 280 airlines, has reacted cautiously.
"Our basic stance is that we do not want guns on board the
aircraft, we certainly don't want the cockpit to be armed,"
IATA spokesman Anthony Concil told AFP news agency.
Airlines would comply, he said, but if the measures are obligatory, then it should be the governments which pay for them.
The American directive, which has come into immediate effect, applies to all flights using American air space. An estimated 800 to 1,000 passenger flights a day could potentially be required to use sky marshals.
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".
Armed marshals disguised as passengers are already deployed on thousands of US flights each week.
Several countries, including Germany and Canada, already use armed guards, while others are considering, or in the process of implementing, the measure.
Australia, which has guards on domestic flights and on flights to Singapore, is working out an agreement with the US authorities for marshals on US-bound flights.
The UK has announced that it is putting undercover armed guards on some flights and KLM Royal Dutch Airlines is in talks with the Dutch Government about the measure.
A spokeswoman for Air France said unarmed security guards had been on board "flights judged to be sensitive" since the 11 September 2001 attacks.
The airline cancelled six flights between Paris and Los Angeles last week.
US intelligence has suggested that al-Qaeda members posing a "credible threat" were due to board.
Just before Christmas, the US stepped up the state of alert in the country in response to a substantial increase in intelligence reports about possible threats.