Two Americans and a British soldier are reported to have been killed and 12 others injured by a rocket attack on a coalition base in Iraq, according to US defence officials.

Within hours airstrikes were reported on an area of the Iraqi-Syrian border used as a base by an Iran-backed militia, raising fears of a fresh round of US-Iranian escalation that brought the countries close to war in January.

Camp Taji, just north of Baghdad, was hit by between 15 and 30 Katyusha rockets on Wednesday evening. The French Press Agency said that three dead were a US soldier, a US contractor and a British soldier.

Col Myles Caggins, a spokesman for Operation Inherent Resolve, the counter-terrorism mission in Iraq and Syria, confirmed on Twitter that “more than 15 small rockets impacted Iraq’s Camp Taji base hosting coalition troops, March 11 at 7.35 pm.”

Boris Johnson described the attack on the Taji base as “deplorable”.

“Our servicemen and women work tirelessly every day to uphold security and stability in the region – their presence makes us all safer,” the prime minister said. “The foreign secretary has spoken to the US secretary of state and we will continue to liaise with our international partners to fully understand the details of this abhorrent attack.”

Soon after the Taji attack, airstrikes by unidentified warplanes were reported south-east of the Syrian town of al-Bukamal, near the border with Iraq. Iraqi state media reported there was only material damage, with no casualties. Reuters quoted western intelligence sources as pointing out that al-Bukamal lies on a strategic supply route for Iranian-backed militias who constitute the Iraqi Shia Popular Mobilisation Forces.

The last time an American was killed in a rocket attack on a coalition base, the Trump administration held Iran responsible and the subsequent tit-for-tat escalation led to the US drone assassination of the Iranian Revolutionary Guard commander, Qassem Suleimani, on 3 January, bringing both countries to the brink of war. At the height of the resulting tension, Iranian air defence mistakenly shot down a Ukrainian civilian airliner, killing 176 people.

Congress passed a bipartisan resolution on Wednesday evening requiring the president to seek congressional authorization before entering a conflict in Iran. Trump has threatened to veto the resolution – a veto which would need a two-thirds majority in the Senate to overcome.

There were no immediate claims of responsibility for Wednesday night’s rocket attack, which came on what would have been Qassem Suleimani’s 63rd birthday. The US administration has repeatedly warned that it would treat an attack by any Iran-backed group in Iraq as a direct attack by Iran itself.

Iran’s immediate response to the Suleimani killing was to launch a volley of missiles at an Iraqi base hosting US soldiers. Since then, Iraq’s parliament has voted to expel foreign soldiers from the country, but the decision has yet to be executed by the government.