The US State Department says it is investigating reports that a US citizen carried out a suicide bombing in Syria.
The American is reported to have been a member of al Nusra Front, a group linked to al Qaeda. Analysts say it would make him the first American to be involved in such an attack.
In a series of tweets, the Islamist rebel faction identified him as Abu Hurayra al Amriki, or Abu Hurayra the American.
The man, whose real name has not been released, is said to have carried out an attack on Sunday in northern Syria, which has seen increased fighting in recent weeks.
State Department spokeswoman Jen Psaki told Thursday's daily press briefing: "We are of course looking into those reports, but cannot confirm anything at this time."
A photo circulated online, purportedly of the bomber, shows a bearded man, smiling and stroking a cat.
Laith Alkhouri, a senior analyst with Flashpoint Global Partners, which monitors militant websites, told Sky News: "This makes him the first US suicide bomber in Syria to have committed the operation on behalf of al Qaeda.
"No other similar claims of responsibility involving an American have been issued previously during this Syrian conflict."
Western officials are worried citizens travelling to Syria from countries such as the United States or Britain could become radicalised and train as terrorists.
Some officials have said such citizens may pose a threat once they return to their home nations.
Around 400 Britons are believed to have gone to Syria over the last two years to train in camps or take part in the fighting, with an estimated 20 having died.
Officials in the UK have advised those who want to support humanitarian efforts in Syria to donate to charities instead of travelling to the country.