At least five members of paramilitary group were killed in US airstrikes on the border region between Iraq and Syria, a war monitor said on Monday.

The Syrian Observatory for Human Rights (SOHR) said the fighters were killed and several others wounded during the strikes on the Syrian side of the frontier, reports Xinhua news agency.

The Pentagon said on Sunday that it conducted airstrikes against facilities used by "Iran-backed militia groups" in the region.

"The US strikes targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq, both of which lie close to the border between those countries," Pentagon Press Secretary John Kirby said in a statement.

According to Kirby, resident Joe Biden directed the airstrikes against Iran-backed militia targets.

It was Biden's second use of force in the region since taking office five months ago, a retaliatory response to what Washington claimed a series of "attacks by Iran-backed groups targeting US interests in Iraq."

In February, at least 17 members of paramilitary group were killed in the US strikes in Syria at the Iraq border, according to the SOHR.

Mary Ellen O'Connell, a professor at Notre Dame Law School, has criticized the US attacks as a violation of international law.

"The UN Charter makes absolutely clear that the use of military force on the territory of a foreign sovereign state is lawful only in response to an armed attack on the defending state for which the target state is responsible," she said.

"None of those elements is met in the Syria strike," she was quoted by media reports as saying.

The airstrikes came at a delicate time between the US and Iran.

Talks on restoring the 2015 Iran nuclear agreement, or the Joint Comprehensive Plan of Action (JCPOA), in Austria's capital Vienna have shown a positive signal of finalising the landmark eal and finding solutions to the Iranian nuclear issue.

During the JCPOA Joint Commission meeting earlier in June, representatives from China, France, Germany, Russia, the UK and Iran agreed on many important issues concerning the revival.

However, the US and Iran remain divided over how to achieve a mutual return to compliance with the nuclear deal.