A Hezbollah fighter fires at a position of the Islamic State (IS) group in Syria's Qalamoun region, on Aug. 26, 2017. (Xinhua/Ammar Safarjalani)

US airstrikes against a pro-Iran group in Iraq killed at least 25 fighters, a paramilitary umbrella said Monday, triggering anger in a country caught up in mounting tensions between Tehran and Washington.

The attacks on Sunday night saw US planes hit several bases belonging to the Hezbollah brigades, one of the most radical factions of Hashed al-Shaabi, a Tehran-­backed Iraqi paramilitary coalition.

The strikes "killed 25 and wounded 51, including commanders and fighters, and the toll could yet rise," according to the Hashed, which holds major sway in Iraq. It said it was still pulling victims from the rubble of bases near Al-Qaim, an Iraqi district bordering Syria, on Monday.

Iran's foreign ministry spokesperson Abbas Mousavi said the US had "shown its firm support for terrorism and its neglect for the independence and sovereignty of countries" by carrying out the attacks. Washington - itself a key ally of Baghdad - must accept the consequences of its "illegal act," Mousavi added.

US Defence Secretary Mark Esper described the attacks against the Hezbollah brigades - which hit three locations in Iraq and two in neighboring Syria - as "successful," and did not rule out further military action against Iran-backed militias.

The strikes were retaliation for a series of rocket attacks since late October against US interests in Iraq, including a barrage of more than 30 fired on Friday against an Iraqi base in Kirkuk, which killed a US civilian contractor. 

US-Iran tensions have soared since Washington pulled out of a multilateral nuclear agreement with Tehran last year and imposed crippling sanctions.

Iraqi leaders fear their country could become a battleground between Tehran and Washington, in a context where they are also grappling with huge street protests against corruption and Iran's enormous political influence.

The demonstrations forced prime minister Abel Abdel Mahdi to resign last month, although he remains in a caretaker role. The protest movement, along with President Barham Salih, have rejected Iran's favored successor.

US sources say that pro-Iran armed factions now pose a greater threat than the Islamic State, a jihadist group whose rise saw the US freshly deploy troops on Iraqi soil.

"China has always opposed the use of force in international relations," Chinese Foreign Ministry spokesperson Geng Shuang said Monday in a regular media briefing.

"We maintain that all parties should earnestly abide by the purposes and principles of the UN Charter and the basic principles of international relations."

Geng pointed out that the sovereignty, independence and territorial integrity of Iraq and Syria should be respected and maintained. 

"We call on all parties concerned to maintain calm and restraint to avoid escalating tensions," Geng said.