Alarmed by an exodus of money from Afghanistan, U.S. and Afghan authorities are trying to constrict a flow of cash through the country's main airport, believed to be a major conduit for drug proceeds and diverted foreign aid.

The plan, part of an effort to monitor an outflow of money thought to exceed $1 billion a year, centers on the installation of U.S.-developed currency counters at the Kabul airport. The devices would be used to record serial numbers on bills to determine whether money being carried out of the country has been siphoned from aid funds flowing in, U.S. and Afghan officials said.

Authorities also said they intend to eliminate an arrangement that has allowed top government officials and other well-connected Afghans to board planes -- even while carrying suitcases packed with cash -- without declaring the transfers or being searched.

The measures represent the latest effort to combat rampant corruption in Afghanistan with a combination of sophisticated U.S. technology and expanded enforcement. U.S. government agencies have also provided Afghan units with wiretap equipment that has been used to build corruption cases against senior government officials in Afghanistan, including a top national security adviser to President Hamid Karzai.

It is legal to carry even large sums of cash out of Afghanistan, as long as the transfers are declared. But authorities have grown increasingly convinced that much of the money is being siphoned from U.S.-backed aid projects and swelling secret accounts being set up by elite Afghans in other countries, including Dubai.