Walid Muallem, the Syrian Foreign Minister, told reporters on Sunday: "This is meddling in Syria's internal affairs and we reject it."
The US State Department said on Friday that it would allocate $5 million to finance the Syrian opposition amid worsening relations between the two countries.
Condoleezza Rice, the US Secretary of State, said on Thursday that the United States wanted to strengthen sanctions against Syria and was trying to convince other nations to follow suit.
The Bush administration backs a United Nations inquiry into the murder a year ago of Rafiq al-Hariri, the former Lebanese prime minister, and 22 other people in a Beirut seafront bombing that has been linked to Syria.
Washington also frequently accuses Syria of failing to stem the flow of foreign fighters across its border with Iraq.