As he called for more troops he said that he would leave "the decision [for troop enhancements] to the military men within Nato".
While Rice and Miliband are in Kabul, Nato members are meeting in the Lithuanian capital, Vilnius, to assess their long-term strategy in Afghanistan.
James Appathurai, a Nato spokesman, told Al Jazeera, that troop contributions are a "long-term effort".
Appathurai was speaking in response to comments by Robert Gates, the US defence secretary, that Nato's input in Afghanistan showed "a lack of commitment".
He said that Nato "is in it for the long-haul", adding that there are today 10,000 more troops in Afghanistan than there were a year ago, confirming that the country currently hosts 43,000 Nato troops.
Appathurai defended the coalition, saying that saving Afghanistan was a "team effort" and that the world's most powerful countries should all work together for a better Afghanistan.
Thursday's surprise visit to Afghanistan comes after Rice and Miliband increased pressure on Nato allies to provide more troops, arguing that the alliance faces a huge battle against the Taliban.
Rice and Miliband also flew to Kandahar, the former Taliban stronghold in the south.
Miliband said international efforts to stabilise Afghanistan were entering a "new phase" which combined military solutions with political and developmental concerns.
Zeina Khodr, Al Jazeera's correspondent in Afghanistan, said Rice and Miliband could be aiming to reassure the Afghan government that Nato remains committed to its mission in the country.
"Nato has been divided - the US wants their allies to deploy more troops in Afghanistan [and] to deploy them in areas where there has been a lot more violence, particularly in the south and in the west," she reported.
The Nato-led International Security Assistance Force has 43,250 troops in Afghanistan. The US has a further 14,000-plus troops outside the Nato force.
A country-wise breakdown:
US: 15,000 troops (mainly east)
UK: 7,800 (mainly south)
Germany: 3,210 (north)
Italy: 2,880 (west and Kabul)
Canada: 2,500 (south)
The Netherlands: 1,650 (south)
France: 1,515 (Kabul)
Poland: 1,100 (mobile)
Australia: 1,070 (south)
Other countries: Denmark (780); Spain (740); Turkey (675); Romania (535)
"Other observers tell us that the Americans would like to bridge the gap between the Afghan and British governments, as ... Hamid Karzai [Afghan president] has openly criticised the British in the past, particularly their policies in Helmand province in the south."
Most of the fighting against the Taliban is currently shouldered by Britain, the United States, Canada and the Netherlands.
Last week, Germany publicly rejected US calls to deploy more troops in the battle scarred south.
Under its parliamentary mandate, Germany can send only 3,500 soldiers to the less dangerous north.
Rice also told reporters that Robert Gates, the US defence secretary, will be working on building troop contributions at a Nato meeting of defence ministers in Vilnius, Lithuania,
Commanders in Afghanistan have been calling for an extra 7,500 troops to battle the Taliban stronghold in the south.