Tim Kaine, the Democratic senator from Virginia, says the Trump administration must receive authorization from Congress before targeting forces loyal to Bashar al Assad

A Democratic senator says the US military's recent strikes against Syrian government targets are 'completely illegal,' it was reported on Wednesday.

Tim Kaine, the Democratic senator from Virginia, told Sirius XM on Wednesday that the Trump administration must receive authorization from Congress before targeting forces loyal to President Bashar al Assad.

'I think the military action that is being taken against Syrian government assets is completely illegal,' said Kaine, who was Hillary Clinton's running mate during the recent election campaign.

Kaine's remarks were made one day after the US-led coalition said it had shot down an armed pro-Syrian drone early Monday morning that had been advancing on its forces near a garrison close to the border with Iraq in southeastern Syria.

Earlier this week, the US military announced it had shot down a Syrian warplane near Raqqa.

The Pentagon has argued that it has legal authorization to use force in Syria.

Kaine (right) and Arizona Senator Jeff Flake (left), a Republican, are urging Congress to reassert its power in granting the administration the authorization to expand its military operations in Syria

The Trump administration, like former President Barack Obama's, has been using a 2001 Authorization for the Use of Military Force (AUMF) against al Qaeda passed after the September 11 attacks as the legal basis for a wide range of military action since.

Kaine says that rationale is flimsy considering that Syria did not play a role in the 9/11 attacks.

'The 2001 authorization said we can take action against the perpetrators of the 9/11 attacks,' Kaine said.

'Nobody claims that Syria was a perpetrator. Nobody claims that they are connected to al Qaeda. In fact, they're battling against al Qaeda in Syria.'

US forces in Syria shot down an armed drone affiliated with groups supporting Syrian President Bashar Assad's government overnight Monday. It happened near Al Tanf, where the US military is using the area near Syria's border with Iraq and Jordan to train its local forces

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.

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.