The United States has carried out another round of airstrikes against Iran-backed militia in Iraq and Syria, the Pentagon has said.

The strikes, which were ordered by President Joe Biden, were in response to drone attacks by the militia against US personnel and facilities in Iraq.

The US military said in a statement it targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq.

It did not disclose whether it believed anyone was killed or injured but officials said assessments were ongoing.

Iraqi militia groups, aligned with Iran, have named four members of the Kataib Sayyed al-Shuhada faction they said were killed in the attack on the Syria-Iraq border.

The militia group vowed to retaliate.

The US strikes mark the second time Mr Biden has ordered retaliatory strikes against Iran-backed militia since taking office five months ago.

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He last ordered limited strikes in Syria in February, that time in response to rocket attacks in Iraq.

“As demonstrated by this evening’s strikes, President Biden has been clear that he will act to protect US personnel,” the Pentagon said in a statement.

The strikes came even as Biden’s administration is looking to potentially revive a 2015 nuclear deal with Iran.

The decision to retaliate appears to show how Mr Biden aims to compartmentalise such defensive strikes, while simultaneously engaging Tehran in diplomacy.

Mr Biden’s critics say Iran cannot be trusted and point to the drone attacks as further evidence that Tehran and its proxies will never accept a US military presence in Iraq or Syria.

The president and the White House declined to comment on the strikes on Sunday.

Mr Biden will meet Reuven Rivlin, Israel’s outgoing president, at the White House on Monday for a broad discussion that will include Iran and US efforts to re-enter the Iran nuclear deal.

Those efforts have raised serious concerns in Israel, Iran’s arch-rival.

US officials believe Iran is behind a ramp-up in increasingly sophisticated drone attacks and periodic rocket fire against its personnel and facilities in Iraq, where the US military has been helping Baghdad combat the remnants of Islamic State.

Two US officials, speaking to Reuters on condition of anonymity, said Iran-backed militias carried out at least five drone attacks against facilities used by U.S. and coalition personnel in Iraq since April.

The Pentagon said the facilities targeted were used by Iran-backed militia including Kataib Hezbollah and Kataib Sayyid

One of the facilities targeted was used to launch and recover the drones, a defence official said.

The US military carried out strikes with F-15 and F-16 aircraft, officials said, adding the pilots made it back from the mission safely.

One of the officials told Reuters: “We assess each strike hit the intended targets.”

Iraq’s government is struggling to deal with militias ideologically aligned with Iran which are accused of rocket fire against US forces and of involvement in killing peaceful pro-democracy activists.

Earlier in June, Iraq released Iran-aligned militia commander Qasim Muslih, who was arrested in May on terrorism-related charges, after authorities found insufficient evidence against him.