The drone was an Iranian-made Shahed 129, thought to be armed in range of US troops (File of American drone pictured)
'So I think this is a completely unlawful use of power.'
In a statement issued on Tuesday, US forces said the drone was shot at after it 'displayed hostile intent and advanced on coalition forces'.
One official said that the drone was shot down because it was 'assessed to be a threat.'
The downing of the drone comes just days after a US F/A-18 shot down a regime warplane after dropping bombs near US-backed Syrian Democratic Forces fighting ISIS in the area around Tabqah, Syria, in the north of country.
The US says an SU-22 operated by the Syrian regime was dropping bombs on the Syrian Democratic Forces when it was shot out of the sky by a U.S. F/A-18E Super Hornet.
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This US Navy file photo shows an F/A-18E Super Hornet. The US says an SU-22 operated by the Syrian regime was dropping bombs on the Syrian Democratic Forces, a group that is aligned with the US, when it was shot out of the sky by a Super Hornet earlier this week
The downing of the Syrian plane angered Damascus' key ally and backer, Russia.
Earlier on Monday, a Russian warplane came within five feet of a US Air Force reconnaissance plane which was flying a mission over the Baltic Sea on Monday.
According to the officials, a Russian Su-27 fighter jet, which was armed with missiles, 'rapidly' approached an American RC-135 reconnaissance plane.
As the aircraft were flying side-by-side, the Russian pilot began to undertake 'provocative' maneuvers by flying 'erratically,' according to a US official.
In April, the US Navy launched dozens of Tomahawk missiles at a Syrian airfield from a ship in the Mediterranean.
The strikes were in retaliation for an alleged chemical weapons attack ordered by the Assad government against Syrians.
Although there is bipartisan support for Congress to debate and vote on a new AUMF introduced by Kaine and Republican Senator Jeff Flake, the measure faces stiff opposition.
Lawmakers have introduced war authorizations repeatedly in the past several years.
But they have failed to advance amid sharp divisions in Congress over whether, or how, to limit commanders' use of military resources.
US senators called on Congress on Tuesday to take back its authority to determine whether the country goes to war, saying recent US strikes in Syria were not covered by existing authorizations for the use of military force.
The Senate Foreign Relations Committee has begun considering legislation that would cover military action in Syria, Afghanistan, Iraq, Somalia, Libya and Yemen against the Islamic State, al Qaeda and other Islamist militant groups.
'I have always believed that it's important for Congress to exercise its constitutional role to authorize the use of force,' the committee's chairman, Republican Senator Bob Corker, told a hearing on Tuesday.