Two American soldiers and a British soldier were killed and about a dozen people were wounded on Wednesday, US officials said, in the deadliest rocket attack in years on an Iraqi military base hosting foreign troops.  The attack threatens a dangerous escalation, with suspected US-led coalition air strikes promptly targeting Iran-aligned Iraqi fighters in neighboring Syria, the Syrian Observatory for Human Rights (SOHR) said. At least 18 fighters were reported killed.  On Wednesday evening, a volley of 18 107 mm Katyusha rockets hit the Taji air base north of Baghdad which hosts troops from the US-led coalition helping local forces battle militants. A coalition statement said three of its personnel were killed and another 12 were wounded but it did not disclose the nationalities of the casualties. US officials, however, told several news agencies that the dead included two Americans and a British national.  The Iraqi military said the rockets were fired from the back of a truck, but it did not comment on any casualties. While there was no immediate claim of responsibility, Washington has blamed Iran-backed factions from Iraqs Popular Mobilization Forces (PMF) military network for recent similar violence. US Secretary of State Mike Pompeo and British Foreign Minister Dominic Raab condemned the attack in a telephone call. The two "underscored that those responsible for the attacks must be held accountable," the US State Department said in a statement. Within hours of Wednesdays attack, three warplanes likely belonging to the US-led coalition bombed PMF factions stationed on the Syrian side of the border with Iraq, according to the SOHR. At least 18 Iraqi fighters were killed as "ten explosions shook the area near Albukamal," a Syrian border town with a heavy PMF presence, said Observatory head Rami Abdel Rahman. US officials said, however, that the United States had not carried out any retaliatory strikes at this point. Wednesdays attack on the Taji air base was the 22nd on US installations in Iraq, including the American embassy, since late October. One of the earlier attacks killed an Iraqi soldier and in late December, a spree of rockets killed a US contractor working at a base in northern Iraq.   That killing sent tensions spiraling. Two days later, the US responded by bombing Kataeb Hezbollah, a faction within PMF that is heavily backed by Iran and which Washington has blamed for several rocket attacks. At least 25 Kataeb Hezbollah fighters were killed and their supporters on 31 December besieged the US embassy in Baghdad.  A US drone strike outside the Baghdad airport on January 3 killed Iranian military commander Qasem Soleimani and the PMFs deputy chief Abu Mahdi al-Muhandis.  Iran then retaliated by launching a volley of missiles at the western Iraqi base of Ain Al-Asad. While no personnel were killed in that attack, dozens of US soldiers suffered from brain trauma.  The tit-for-tat attacks on Iraqi soil left Baghdad furious, and in January its parliament voted to expel all foreign soldiers from Iraq in reaction to the killing of Soleimani.  Among them are some 5,200 US forces stationed across Iraq as part of the international coalition -- comprised of dozens of countries -- formed in 2014 to confront ISIS. While ISIS has lost its territory, sleeper cells remain capable of carrying out attacks.  On Sunday, two US soldiers were killed while helping Iraqi forces battle ISIS remnants north of Baghdad.