KABUL - U.S.-funded development firms are beginning to shut down massive reconstruction projects because the Afghan government has refused to rescind a ban on their use of private security guards, according to U.S. officials and aid workers here.

The decision to start shuttering the projects, collectively worth hundreds of millions of dollars, could have far-reaching effects on the U.S.-led military campaign against the Taliban, disrupting a central component of the strategy to counter the insurgency at a critical moment in the war. Programs to assist Afghan farmers and improve local government, which are vital to the overall U.S. effort to stabilize the volatile southern and eastern parts of the country, are among those that will be affected, the officials said.

The consequences of the ban on development firms employing private guards "will be catastrophic," said one U.S. official involved in the issue. "If these projects grind to a halt, we might as well go home. They are essential to the counterinsurgency strategy."

Another U.S. official said the ban would affect about $1.5 billion in ongoing reconstruction work. More than 20,000 Afghans will lose jobs in road-building and energy projects alone, the official said.

The prohibition, which was enacted by President Hamid Karzai, has emerged as the latest flashpoint in the oft-strained U.S.-Afghan relationship, raising new questions about his willingness to cooperate with the international community and potentially complicating crucial year-end assessments of the war effort by the White House and NATO.

The ban, which goes into effect Dec. 17, affects all development firms and non-governmental organizations, including those funded by other countries and the United Nations. It also applies to private contractors who guard supply convoys for the military bearing food, fuel and other essential supplies, as well as to international banks and other private entities whose services support reconstruction work.

Diplomats and representatives of the companies contend that private security guards are essential to operations in much of the country because of the threat of insurgent attacks and criminal activity. But Afghan officials argue that private security contractors operate with little oversight or accountability and sometimes function as private militias that are beyond the control of the country's police force and army.

Most of the private security companies employed by development firms rely on Afghan guards who are supervised by foreigners. Some of the firms are Afghan-owned, while others are international firms that operate in multiple countries.

The Afghan government wants development workers and their projects to be guarded by police officers and soldiers, a goal that diplomats and aid workers say is unrealistic because the local government security forces are corrupt, ill-trained and not numerous enough to do the job. The development firms have also said they would be unable to insure their employees - a key prerequisite for operations in a war zone - if they are unable to employ private guards.

"If we don't have private security, we cannot operate in Afghanistan," said one development executive, who, like others, spoke on the condition of anonymity because of the sensitivity of the issue. "It's not open to negotiation."

U.S. and NATO diplomats have been working furiously to soften the prohibition since it was authorized over the summer. Under pressure from Gen. David H. Petraeus, the top U.S. and NATO commander here, Karzai's national security council issued an exemption Sunday for military bases, embassies, diplomatic residences and diplomatic convoys.

Diplomats and aid workers initially assumed that the failure to mention development organizations was an oversight that would be corrected. But Karzai on Wednesday said the ban would not be amended. "These companies are closed - that is it," he said, referring to security firms not working for diplomats or the military.