The United States and its allies should immediately announce how many troops will stay on in Afghanistan after 2014, former NATO supreme commander Admiral James Stavridis argued on Wednesday.

Stavridis, who recently finished a four-year stint as the alliance's top military leader, said it was vital to unveil the troop plans quickly to counter Taliban propaganda claiming foreign troops are abandoning the country.

In a commentary, Stavridis wrote that he supported keeping 15,000 US and allied forces in the country after the bulk of coalition troops withdraw as planned in 2014.

"I believe the correct number is about 9,000 US and 6,000 allied troops, for a total of about 15,000 allied trainers who would focus on mentoring, training, and advising the 350,000 strong Afghan National Security Forces," Stavridis wrote in Foreign Policy.

"Instead of waiting for months, we should move now to decide and publicly reveal the commitment," the admiral said.

There are now roughly 100,000 troops in Afghanistan in the NATO-led coalition, with the Americans making up about two-thirds of the force.

US officials have long suggested they expected to retain a smaller force of about 8,000 to 12,000 troops after 2014.

But, amid difficult negotiations on a long-term security accord with Kabul, White House officials have spoken of a possible "zero option" with no US boots on the ground after next year.

The former top commander of US and alliance forces in Afghanistan, General John Allen, has called for keeping 13,600 American troops in the country, as well as a number of NATO forces. He too has urged President Barack Obama to announce the troop decision as soon as possible.

By announcing troop plans now, Stavridis said the allies "would break the Taliban narrative decisively, making a lie of their oft-repeated trope that 'the foreigners are leaving,'" the admiral wrote.

The move "would reassure the Afghans" and "demonstrate needed leadership to the large international coalition that is awaiting US decisions," he wrote.

"It would also encourage the conclusion of the strategic agreement between the United States and Afghanistan, " he added.

The current head of the International Security Assistance Force (ISAF) in Afghanistan, General Joe Dunford, has indicated he would provide recommendations on troop numbers after this year's "fighting season" ends in October.

While acknowledging serious difficulties in the war effort, Stavridis defined "success" in Afghanistan as a "democratic (if somewhat corrupt) nation that has reasonable control over its borders" and dominance over an insurgency that does not pose a dire threat to the state.

If 15,000 allied troops stay in Afghanistan, there was cause to be "cautiously optimistic" the country would meet the criteria of success, he wrote.

But if no foreign troops remain after 2014, that would mean "probable mission failure," according to Stavridis.