The PM said the UK was doing all it could to make Britain's borders secure, as migrants told the BBC they remained determined to reach Britain.
Mr Cameron was speaking after people gathered for a third night at fencing at the Channel Tunnel freight terminal.
More than 3,500 people have tried to get into the tunnel terminal this week.
Several hundred migrants were escorted away from the terminal by French police on Wednesday night - the third night of large-scale attempts to storm the terminal.
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At the scene
Lucy Williamson, BBC correspondent:
As night fell, the road towards the Channel Tunnel started to come alive. Groups of 10 or 12 migrants, moving steadily along the darkened highway, jackets pulled close, hoods up.
After a day of discussion in Paris and in London over how to secure the tunnel entrance, and fresh deployments of riot police, the determination of Calais' migrants seems unchanged.
People like Jamal - an Ethiopian who arrived here on Wednesday morning.
He told me he'd spent most of his adult life doing military service in the Ethiopian army, had spent 10 days drifting in the Mediterranean Sea, and had crossed six different European countries to get here.
Not once had his dream of reaching England wavered. For Jamal, a barbed wire fence, or a brush with police, might change his tactics, but probably not his goal.
The fresh attempts came despite the death of a man, believed to be a Sudanese national and aged between 25 and 30, on Tuesday.
French police said he was probably crushed by a lorry which was leaving one of the shuttles that transport vehicles through the tunnel.
Nine people have been killed attempting to cross the Channel in the last month.
Speaking in Vietnam during his tour of South East Asia, Mr Cameron said the French had sent an extra 120 police to Calais and the UK was investing in fencing and security measures at the Channel crossings in both Calais and Coquelles.
"Everything that can be done will be done to make sure our borders are secure and make sure that British holidaymakers are able to go on their holidays," he told the BBC.
The prime minister acknowledged the situation was "very testing" but insisted the government was "totally focused" on the problem, with "intensive" talks going on.
"We have to deal with the problem at source - and that is stopping so many people from travelling across the Mediterranean in search of a better life," he said.
He warned that illegal immigrants would be removed from the UK "so people know it's not a safe haven".
Raihan Jan, 24, a clerk from eastern Afghanistan's Nangarhar province, said he had travelled first to Iran, then on to Turkey, Greece and Italy before arriving in Calais four days ago.
"We heard that one guy died last night and we know it's very dangerous, but there is not another way to go the UK," he said.
"This is the last chance that we have. We think that the economy is a little bit better in the UK so we will get a chance to have documents and some work."
English literature graduate Mohammad Al-Mohammad, 26, said he had fled civil war in Syria. He said he had walked and hitchhiked from a refugee camp in Turkey, through Greece, Macedonia, Serbia, Austria and Italy before arriving in France three months ago.
"I have tried maybe nine or 10 times to get to the tunnel but I have failed," said Mr Al-Mohammad, from Aleppo.
"I am seeking peace in the United Kingdom and my brother is there. I ask the United Kingdom authorities to help me to go to him."
The rail shuttle has seen delays in both the UK and France in recent days, but Eurotunnel said its passenger service was currently operating to schedule.
But disruption to freight traffic looks set to continue, with Operation Stack - where lorries are park on the M20 in Kent when Channel crossings are disrupted - due to run into the weekend.
The UK has brought in a temporary relaxation of the rules that limit how long lorry drivers can drive for and how long they must rest to make it easier for businesses caught up in the disruption.
The BBC's Gavin Lee said migrants in Calais had told him they would keep trying to get through a number of holes in security fences. They told him that going in groups of up to 400 gave them the best chance of getting into the Channel Tunnel.
Eurotunnel said it had blocked 37,000 migrants trying to make their way to Britain since the beginning of the year.
Spokesman John Keefe told the BBC the situation was intolerable, adding: "It is a nightly assault by hundreds, if not thousands, of migrants who are trying to get to the UK by any means possible.
"So they're coming through the fences, trying to get to the freight terminal, they're trying to get on to trucks or trains that would eventually carry them to the UK."
UKIP leader Nigel Farage is among a growing number of politicians who have called for the government to urge France to do more to resolve the situation.
He warned unless "something radical is done it is only a matter of time" before a British holidaymaker or lorry driver died as a result of the crisis.
Labour's interim leader Harriet Harman called on the government to "get a grip" on the situation.
Meanwhile, Labour MP Keith Vaz, who chairs the home affairs committee, urged Mr Cameron to hold talks with French President Francois Hollande on his return from Asia.
