"In counterinsurgency, the human terrain is the decisive terrain, and you must do everything possible to reduce civilian casualties ... in the course of military operations," he said.
The current rules, which were brought in Petraeus's predecessor General Stanley McChrystal, came amid growing Afghan anger at the number of civilian deaths during US and Nato operations.
McChrystal was relieved of his post last week after an article was published in Rolling Stone magazine in which he and his aides criticised officials in the US government.
Zeina Khodr, Al Jazeera's correspondent in Afghanistan, said the government there had welcomed the change.
"Petraeus said the war effort will have to be a combined one - military effort and a civilian one [which will please the government]," she said.
"A lot of Afghans would have welcomed the fact that they heard Petraeus as well as the Nato secretary-general stress the word 'committed', as a lot of people have been hearing over recent weeks about exit plans and withdrawal dates."
William Hague, the new British foreign secretary, said on Thursday he would be "very surprised" if Afghan forces had not taken control of their own security from international forces by 2014.
However, he insisted Britain was not setting a timetable for withdrawing its 9,500 troops from Afghanistan, even after David Cameron, the UK prime minister, said last week that he wanted them home before the next election in 2015.
"There is a difference between all nations," Michael Codner, a defence specialist at the Royal United Services Institute in London, told Al Jazeera.
"US president Obama mentioned last autumn that July was the time to consider troop withdrawals. The UK's David Cameron talked about 2015 would be the time in which they'd hope to reduce, but there other nations such as Canada and the Netherlands who have decided to reduce their forces.
"The smaller contributors would see it as doing their share, and when have done their share then other nations should take it on."
Petraeus was likely during his Brussels visit to have had to smooth ruffled feathers among the European allies who contribute troops to the 122,000-strong international force in Afghanistan.
'Change of command'
Diplomats said member governments were not consulted about the changeover in command after Barack Obama, the US president, suddenly dismissed General Stanley McChrystal, Petraeus' predecessor.
Anders Fogh Rasmussen, Nato's secretary-general, expressed support for McChrystal after the remarks he made to Rolling Stone, and had to backtrack the following day and give his backing to Obama's decision to replace him with Petraeus.
On Thursday, Fogh Rasmussen emphasised his backing of Petraeus.
"This has been a change of command, not a change of strategy," he said. "General Petraeus has our full support."
McChrystal's sacking came amid growing disillusionment with the war in Europe, and increased deaths among foreign troops.
Allied deaths have doubled in the first six months of this year, with June being the deadliest month on record for Nato troops in the nine-year conflict.
Patrick Hennessey, an author and a former soldier who served in Afghanistan and Iraq, told Al Jazeera: "June is traditionally the start of the fighting season proper but the difference between this June and previous ones is that there are far more coalition forces in the south where the fighting is most intense than there has been previously.
"It's the nature of counter insurgencies of the type that Stanely McChrystal had started turning Isaf towards and that General Petraeus is now clearly going to continue, that the soldiers are exposed to more risk but I don’t think we can read that progress hasn't been met because of these casualty numbers," he said.
Accusations of corruption against Afghan government officials and shifting political allegiances in the country have left many countries supporting the war effort uneasy.
Officials said that Petraeus would travel to Afghanistan after the Nato briefing.