Two British soldiers injured in Syria by IS missile
The Syrian Observatory for Human Rights said the pair were part of the international anti-jihadist coalition, led by the United States.
"The two British soldiers were transported by helicopter to receive medical care," the Observatory's director Rami Abdel Rahman said.
A Kurdish fighter from the Syrian Democratic Forces (SDF) died in the attack in the village of Al-Shaafa in Deir az-Zour province, one of the last pockets of territory still controlled by IS in the Euphrates River valley.
The SDF, a coalition dominated by Kurdish fighters, has spearheaded the fight against IS, supported by several Western countries including the United Kingdom.
The international alliance seized the key IS holdout of Hajin in December after months of fighting that has seen the jihadists launch vicious counter-attacks.
IS, which once controlled swathes of Syria and Iraq, has been pounded by multiple offensives.
Since September, more than 1,000 militants have been killed in the fighting compared with just under 600 SDF members while 15,000 people have fled Hajin, according to the Observatory.
Last month US President Donald Trump announced the withdrawal of around 2,000 soldiers from Syria, deployed to support the SDF, claiming IS had been defeated.
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The Syrian war began when the Baath regime, in power since 1963 and led by President Bashar al-Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.
At least tens of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.
The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.