by Helen Corbett, Weedah Hamzah, Kadhem al-Attabi, and Nehal El-Sherif

Syria – Pro-Iranian Shiite militias bombed a US army base in eastern Syria, hours after US airstrikes hit their posts on the Syrian-Iraqi border, a monitor group said late Monday.

The UK-based Syrian Observatory for Human Rights said several rockets hit al-Omar Oil field in Deir al-Zour province, which caused only material damage.

Local sources said that a total of four rockets had been fired from an area controlled by the Syrian government.

The state-run Syrian state news agency SANA also reported the attack.

Russia Is the US changing its nuclear force policy in response to Russia easing its own strike threshold?

Iraq condemned Monday's US airstrikes on Iranian-backed militias on the Iraqi-Syrian border, which killed several people.

The air attack was "a blatant and unacceptable violation of Iraqi sovereignty and national security," armed forces spokesperson Yehia Rasool said. Iraq's Foreign Ministry criticized the strikes in similar terms.

In his statement, Rasool also called for calm and the need to avoid any escalation in the situation.

The Pentagon said on Sunday that US President Joe Biden had ordered the airstrikes on Iran-backed militia groups in the border region.

The facilities were used by Iran-backed militias engaged in drone attacks against US personnel and facilities in Iraq, Pentagon Press Secretary John Kirby said.

Two locations in Syria and one in Iraq – both close to the border between the two countries – were targeted.

China Dozens of Hong Kong democracy campaigners sentenced in crackdown that sparks outrage

Iraq's Popular Mobilization Forces, also known as Hashd al-Shaabi – an umbrella group in Iraq of Iran-backed militias – said four of its fighters were killed in the strikes.

US planes targeted three of its posts eight miles away from the border with Syria, in Anbar province, at 2:00 AM, the group said.

Its forces are stationed in the area to prevent infiltration of Islamic State elements from Syria into Iraq, the group said, and denied there were any depots at these posts.

"We denounce and condemn this attack on our forces in the strongest terms ... and reserve our legal right to respond to such attacks and hold their perpetrators on Iraqi territory accountable," Hashd al-Shaabi said in a statement.

Earlier on Monday, the observatory said seven were killed and others were injured in the strikes, which targeted posts in Syria's eastern countryside.

The Pentagon said the locations were operational and weapons storage facilities used by several Iran-backed groups, including Kata'ib Hezbollah and Kata'ib Sayyid al-Shuhada.

"As demonstrated by this evening's strikes, President Biden has been clear that he will act to protect US personnel. Given the ongoing series of attacks by Iran-backed groups targeting US interests in Iraq, the president directed further military action to disrupt and deter such attacks," Kirby said.

In his first order for a military retaliation as president in February, Biden ordered strikes on Iranian-backed militant groups in eastern Syria in response to rocket attacks on US and coalition personnel at a base in Iraq.