Relations between the US and Argentina have deteriorated after Buenos Aires lodged a formal complaint over a US military plane that landed late last week carrying guns, drugs and satellite phones.
The Argentinian government claimed the US was trying to sneak the weapons into the country, though it didn't offer an explanation of why Washington might want to do this.
The US state department said the consignment was intended for a police training programme in Argentina.
Officials from Argentina and the state department have been in talks aimed at resolving the row.
The relationship between the two countries has been poor since Barack Obama released details of a Latin American tour next month that includes Brazil, Chile and El Salvador but not Argentina.
The Argentinian foreign ministry, in a statement on Sunday night, said "sensitive material" had been seized that had not been declared on the inventory submitted by the US embassy, including weapons, GPS equipment and drugs such as morphine.