(AFP) - The United States launched overnight air strikes against Iran-backed groups in Iraq and Syria, drawing condemnation from both governments Saturday, and promised more to come in retaliation for a deadly attack on US troops.

The United States blamed Sunday's drone attack on a US base in Jordan on forces backed by Iran, but did not strike inside Iranian territory, with both Washington and Tehran seemingly keen to avoid all-out war.

But with tensions in the region already running high in the face of the Israel-Hamas war in Gaza, both Damascus and Baghdad joined Tehran in accusing Washington of undermining the stability of the whole region.

National Security Council spokesman John Kirby said US warplanes struck "more than 85 targets at seven facilities utilised by Iran's Islamic Revolutionary Guard Corps and the militant groups that they sponsor". Three of the facilities were in Iraq and four were in Syria.

"These targets were carefully selected to avoid civilian casualties," he added.

But Iraqi government spokesman Bassem al-Awadi said civilians were among at least 16 people killed in the US strikes in western Iraq.

"This aggressive air strike will push the security situation in Iraq and the region to the brink of the abyss," Awadi said.

Prime Minister Mohamed Shia al-Sudani declared three days of mourning over the deaths.

Iranian foreign ministry spokesman Nasser Kanani said the overnight operation was "another strategic mistake by the US government, which will have no result other than intensifying tension and instability."

Hamas, whose unprecedented October 7 attack on Israel sparked the current spiral of violence in the region, accused Washington of pouring "oil on the fire".

Meanwhile, Russia's foreign ministry accused Washington of "sowing chaos and destruction" in the Middle East.

The Syrian foreign ministry said the strikes served to "inflame the conflict in the Middle East in an extremely dangerous way".

- Flurry of attacks -

The Syrian army said "a number of civilians and soldiers" were killed in the strikes in eastern Syria, but the Syrian Observatory for Human Rights war monitor reported no civilian deaths.

The Britain-based Observatory said the strikes killed 23 pro-Iran fighters, including two from Lebanese Hezbollah, and others were now evacuating their positions for fear of more US strikes.

The Observatory also said civilians in the towns of Deir Ezzor and Mayadeen had fled their homes in fear of fresh US strikes.

US President Joe Biden underlined that the overnight strikes were only a beginning. "Our response began today. It will continue at times and places of our choosing," he said in a statement.

His National Security Council spokesman said Washington "did inform the Iraqi government prior to the strikes," but his remark drew an angry denial from Baghdad.

The Iraqi government spokesman called it an "unfounded claim crafted to mislead international public opinion" and the foreign ministry said it would call in the US charge d'affaires in Baghdad to deliver a formal protest.

Tensions between the two governments have deepened in recent months after Washington carried out previous air strikes against Iran-backed groups in Iraq in response to a flurry of attacks on US-led troops since the Gaza war began last October.

Washington and Baghdad opened talks on the future of the US-led troop presence late last month after repeated demands from Sudani for a timetable for their withdrawal.

- 'Significant escalation' -

The United States has some 900 troops in Syria and 2,500 in Iraq as part of an international coalition against the Islamic State group, which once controlled swathes of both countries.

Its troops in Iraq are deployed at the invitation of Baghdad, but those in Syria are deployed in areas outside the control of the Damascus government.

They operate out of bases in the Kurdish-held northeast or in a small pocket of territory along the borders with Iraq and Jordan.

The Syrian military demanded on Saturday that Washington withdraw its troops.

"The occupation of parts of Syrian territory by US forces cannot continue," it said.

Analysts said the US strikes were unlikely to stem the flurry of attacks on US targets sparked by American support for Israel in its war on Hamas.

The strikes represent a "significant escalation," according to Allison McManus, managing director for national security and international policy at the Center for American Progress.

But she was sceptical about their impact, adding: "We have not seen that similar tit-for-tat strikes have had a deterrent effect."

US and coalition troops have been attacked more than 165 times in Iraq, Syria and Jordan since mid-October.

The soldiers killed Sunday were the first American military deaths from hostile fire in the upsurge of violence.

© Agence France-Presse