He told the Commons liaison committee he could not give a “cast iron guarantee” the target would be met.
But he rejected calls from MPs for it to be dropped.
Net migration – the number coming to the UK for at least a year, minus the numbers leaving – rose 58,000 to 212,000 in the year to September 2013.
Mr Cameron conceded he had no control over some of the key factors feeding into the net migration figure, such as the number of people emigrating from the UK, but he said: “My commitment is to do everything I can to make progress towards that target and that’s what we should do.”
He said the difference between people leaving to work in other EU countries and coming to the UK had been “in broad balance” over time “so the way you get net migration down is by reducing migration from outside the EU” which he said was down by a third since the coalition came to power.
The first official figures on the number of migrants to have arrived from Bulgaria and Romania since restrictions were lifted on 1 January are due to be released on Wednesday.
Chart showing migration figures 2004-13
Home Affairs Committee chairman Keith Vaz told Mr Cameron said the figures would show that 30,000 people had arrived in the UK from those countries.
Mr Cameron said he did not want to comment ahead of the figures, but said it was “remarkable” how many EU migrants had come from countries like Spain and Italy, rather than new EU states in Eastern Europe.
Quizzed by Mr Vaz about the difference between his stance on EU migration and that of UKIP leader Nigel Farage, who wants to end free movement into the UK and reintroduce a work permit system, he said: “I’m not entirely sure what his view is.
“It’s sort of just ‘put the barricades up and pretend that these organisations don’t work’.”
In March, Conservative MP Mark Field called for the Conservative Party to drop its net migration target, saying it was “impractical”, harmed Britain’s global competitiveness and was clearly not going to be met.
Lib Dem business secretary Vince Cable has also disowned the target, stressing that it was a Conservative and not a coalition government policy.
Mr Cameron also agreed to look at a proposal by Bernard Jenkin, Tory chairman of the public administration committee, to improve the system for estimating net migration.
At the moment it is based on a survey of passengers carried out at airports and ports. Plans to count all migrants in and out through an e-borders scheme have been “downgraded”.
Mr Cameron conceded that the existing system has got “inadequacies because it is a survey” but experts, including the Office for National Statistics, had said it was the best way of measuring migration.
He said people were increasingly being counted out as they went through passport control and full exit checks would be in place by next year – but he would examine Mr Jenkins’ call for an extra £15m to improve the collection of migration figures.
Labour’s Shadow Immigration Minister David Hanson said Mr Cameron’s comments showed the Conservatives were “failing” on immigration.
“No amount of bluster from David Cameron will change the fact that he promised categorically to get net migration down to the tens of thousands but it is actually rising, not falling.”