"These are not steps the president takes lightly," said Ben Rhodes, Obama's deputy national security adviser. Rhodes said the president made the decision to authorize military aid over the past few weeks.
The administration announced Thursday that it believes the Syrian regime used chemical weapons. Obama had previously said that condition would constitute crossing a "red line" that would prompt U.S. action.
The full scope of the assistance authorized by the White House is still unclear. But the administration could give the rebels a range of weapons, including small arms, assault rifles, shoulder-fired rocket-propelled grenades and other anti-tank missiles. The opposition forces could operate most of that equipment without significant training.
The Syrian Foreign Ministry said Friday: "The White House has issued a statement full of lies about the use of chemical weapons in Syria based on fabricated information. The United States is using cheap tactics to justify President Barack Obama's decision to arm the Syrian opposition."
Russian authorities said they were not convinced of the White House's claim, that the information provided to Russia by the U.S. "didn't look convincing."
The commander of the main Western-backed rebel group fighting in Syria said he hoped that U.S. weapons will be in the hands of rebels in the near future, noting it would boost the spirits of the fighters on the ground. "We hope to have the weapons and ammunition that we need in the near future," Gen. Salim Idris told Al-Arabiya TV.
Administration officials are also worried about high-powered weapons ending up in the hands of terrorist groups. Hezbollah fighters are among those backing Assad's armed forces, and al-Qaida-linked extremists back the rebellion.
The CIA and special operations trainers are already running some weapons training programs for the rebels and are expected to take charge of teaching the opposition how to use the weapons the U.S. has agreed to supply, another U.S. official said.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies. The administration has also agreed in principle to provide body armor and other equipment such as night-vision goggles to the rebels, although the Pentagon has said there has been no movement on that as yet.
Obama advisers believe Assad's regime still maintains control of Syria's chemical weapons stockpiles and does not see any evidence that rebel forces have launched attacks using the deadly agents. However, British Prime Minister David Cameron said Friday that there were some indications that "al Qaida-linked elements" in the opposition have "attempted to acquire chemical weapons for probable use in Syria."
U.S. Ambassador to the United Nations Susan Rice sent a letter to U.N. Secretary-General Ban Ki-moon detailing claims of use of chemical agent sarin by Syria in Aleppo.
But Ban said Friday he opposes the U.S. decision to send weapons and that there can be no certainty of chemical weapons use in Syria without an on-the-ground investigation. The U.N. chief reiterated his longstanding position that there is no military solution to Syria's two-year-old conflict, which has killed more than 93,000 people. He said increasing the flow of weapons to either side "would not be helpful."
Rice, who will become Obama's national security adviser in July, told reporters Friday that the U.S. government is "very confident" in its assessment.
Hezbollah says it will keep fighting in Syria
Hezbollah's leader vowed Friday that his militants would keep fighting in Syria "wherever needed" after the U.S. agreed to arm the rebels in the civil war, setting up a proxy fight between Iran and the West that threatens to engulf more of the Middle East.
Hezbollah has come under harsh criticism at home and abroad for sending its fighters to Qusair, and Nasrallah's gamble in Syria primarily stems from his group's vested interest in the regime's survival. The Syrian government has been one of Hezbollah's strongest backers for decades, and the militant group fears that if Assad's regime falls, it will be replaced by a U.S.-backed government that is hostile to Hezbollah.
The Associated Press contributed to this report.