The safety of passengers "must come first", David Cameron said as security was stepped up at British airports amid fears that terrorists are working on a bomb that could sidestep current measures.
The Prime Minister said he hoped the measures would not cause unnecessary delays but stressed that no risks could be taken with passenger safety.
Changes to security measures were announced after Washington homeland security secretary Jeh Johnson ordered beefed-up security at foreign airports which have direct flights to the US, reportedly as a result of intelligence that groups in Yemen and Syria had joined forces to plot an attack.
Mr Cameron told the BBC: "W e take these decisions looking at the evidence in front of us and working with our partners.
"This is something we've discussed with the Americans and what we have done is put in place some extra precautions and extra checks.
"The safety of the travelling public must come first. We mustn't take any risks with that.
"I hope this won't lead to unnecessary delays but it's very important that we always put safety first, and we do."
The Government highlighted the importance of vigilance but said the extra measures - which have not been disclosed - are not expected to cause "significant disruption" to passengers and the official UK threat status remains unchanged at "substantial", the third grade in the five-level rating.
A Department for Transport spokesman said: " We have taken the decision to step up some of our aviation security measures. For obvious reasons, we will not be commenting in detail on those changes.
"The majority of passengers should not experience significant disruption."
Deputy Prime Minister Nick Clegg cautioned that the new checks were unlikely to be "a one-off, temporary thing".
"This is the world we now live in," he said on his weekly LBC radio phone-in.
"I don't want people to think that this is some sort of blip for a week. This is part of an evolving and constant review about whether the checks in our airports - and indeed other places of entry and exits from countries - keep up with what we know from intelligence and other sources about the nature of the threats we face."
The increased security measures - reported to include closer scrutiny of personal electronics and footwear - come amid fears that individuals with Western passports who have travelled to Syria and Iraq to fight with Islamist extremists could be used to smuggle devices on to planes.
Intelligence is reported to suggest bomb-makers from Yemen-based al Qaida in the Arabian peninsula (AQAP) have travelled to Syria to meet al Qaida-affiliated Jabhat al-Nusra to work on ways to get an explosive device past existing security.
Al Qaida operatives in Yemen were behind an underwear bomb that failed to detonate aboard a jet over Detroit on Christmas Day 2009 and a more sophisticated version which was intercepted in a CIA sting operation three years later.
Saudi-born bomb-maker Ibrahim Hassan al-Asiri, believed to be a member of AQAP, was also said to be behind a powerful bomb hidden in printer ink cartridges which was intercepted at a UK airport en route to the US in 2010, where it was timed to detonate over the east coast.
Downing Street said there was an "evolving threat" and people should continue to fly but allow "appropriate time" to go through security.
The Prime Minister's official spokesman said passengers "will understand why it is necessary to have security measures in place" and added: "The advice from all of government would be for people to continue to follow the advice from airports and airlines.
"That includes, as it has done for a very long time, giving themselves the appropriate time to go through all the airport procedures."
Asked whether airports and airlines should increase staffing levels to avoid delays, the spokesman said: "We will continue to work with the aviation sector, airlines and airports, on this.
"The industry has a strong incentive to minimise any impact on passengers as far as possible, of course that is the case. I'm sure they will continue to do that."
Transport Secretary Patrick McLoughlin said: " I would like to reassure the travelling public that we have one of the toughest security regimes in the world, along with the US."
The tightening of security came as a message was posted on a Twitter page purporting to be that of a British jihadi in Iraq, saying the UK was "afraid" he might come back with skills he had learned fighting with terror group the Islamic State in Iraq and Greater Syria (Isis).
The message was posted yesterday on an account believed to belong to Nasser Muthana, from Cardiff, who appeared in an Isis propaganda video released last week.
He tweeted, alongside a picture of containers: "So the UK is afraid I come back with the skills I've gained."
The Foreign Office updated its travel advice on Uganda after the authorities in Kampala warned of a possible terror threat against Entebbe Airport today.
The advice reads: " The Ugandan authorities remain concerned about the possibility of a further attack and have issued a number of alerts warning of a heightened risk of terrorism, including most recently at the beginning of July, about a possible threat against Entebbe airport on 3 July 2014.
"There may also be additional security checks, including baggage searches, on the approach to Entebbe Airport."