KABUL, Afghanistan  The NATO military command in Afghanistan has quietly issued new guidelines on contracting aimed at pushing commanders to take a tougher approach to how they spend billions of dollars. It is part of an effort to reduce corruption and decrease the funds that indirectly flow to the insurgency.

The American-led military operation here now spends about $14 billion a year on contracting work, according to military officials.

The new guidelines, issued last week by Gen. David H. Petraeus, the top military leader here, make clear that he wants his subordinates to use their purchasing power as a tool in the fight against the insurgency and that so far, that has not happened.

The provisions in the new guidelines suggest that on many occasions NATO contracts have ended up exacerbating Afghanistan’s problems by empowering warlords and entrenched power brokers, and undercutting the trust of average Afghans, who see dollars being spent but never reap the benefits.