The Taliban "lack the capacity to hold ground," he wrote, in response to claims that Taliban fighters have drawn a noose around the capital Kabul.
Some of the insecurity in Afgahnistan was due to "growing criminality", including the trade in heroin, Miliband said.
He welcomed the move by defence ministers within the Nato military alliance to allow their forces to work closer with Afghan security forces in order to crack down on drug facilities.
Pointing to the fact that 18 provinces in Afghanistan have been drug-free this year, up from 13 last year, he said there had been "progress. Not enough, but progress all the same."
Miliband said that Asif Ali Zardari, Pakistan's president, had pledged his co-operation to the international effort in Afghanistan, and that so far "he has been true to his word".
The tribal areas of Pakistan are said by the US to be a base for fighters who launch attacks in Afghanistan.
“The role of the international community is not to wring its hands and go home, but to help the Pakistan government get a grip on its tribal areas," Miliband said.
A "massive economic effort with the help of the international community" is requited to help secure the Afghan-Pakistani border regions in addition to "doing better" against suicide attacks, Miliband wrote.
About 7,800 British troops are serving in Afghanistan as part of the Nato-led International Security Assistance Force (Isaf) and US-led operations.