US Defence Secretary Leon Panetta urged the international community on Saturday to help pay for strong Afghan security forces despite worldwide economic pressure.

The United States is spending around $12 billion a year (2.3 billion euros) to train the Afghan security force (ANSF), which is expected to rise to 352,000 men in order to take over security when NATO combat troops withdraw at the end of 2014.

"To sustain sufficient security, the ANSF requires adequate financial support," Panetta said in a speech in Munich, recalling that the international community committed to helping Afghanistan at a Bonn conference in December.

The United States has forecast that the annual price tag of training and equipping Afghan security forces in coming years to be around $6 billion.

Washington wants the international community to contribute $1 billion per year after 2014 in addition to the US share, said a senior US defence official.

"I know we face intense pressure to reduce that support given the budget constraints all ISAF nations are facing," Panetta said, referring to the 50 countries in the NATO-led International Security Assistance Force.

"But even as we will work to find ways to reduce ANSF costs over time, we cannot shortchange our commitment, nor count on other nations to fill the gap. We must do everything we can to support this force."

The long-term size of the Afghan force and cost of maintaining it will be a key topic at a NATO summit in Chicago in May.

NATO defence ministers voiced hope during talks on Thursday and Friday that Afghan forces can take the lead across the country in 2013, while foreign troops shift to a backup role.

Panetta had caused a stir before arriving in Europe on Wednesday when he suggested that the United States hoped to wind down the combat mission as early as mid-2013.

But he has since insisted that US troops would still be involved in combat through 2014, a point he emphasised on Saturday at the Munich Security Conference, a gathering of world leaders and security experts.

"Based on progress in the Afghan forces, we believe they will be ready to take the combat lead in all of Afghanistan some time in 2013. When they do, we will shift naturally to a support role," he said.

"Of course, ISAF will continue to be fully combat capable, and we will engage in combat alongside the Afghans as necessary thereafter," he said.