The deputy leader of al-Qaeda was the target of an air strike in the Syrian province of Idlib on Sunday, according to unconfirmed reports.
It is not clear whether Abu al-Khayr al-Masri survived the attack.
The Egyptian is second-in-command to overall al-Qaeda leader Ayman al-Zawahiri.
Syrian opposition activists, the Local Co-ordination Committees, posted photographs showing a car in which two people had been killed, it said.
The car, in the town of al-Mastuma, was targeted by "international coalition aircraft", the group said.
The Syrian Observatory for Human Rights reported that an al-Qaeda official was killed in a strike, but did not confirm it was Abu al-Khayr al-Masri.
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The Egyptian, whose real name is Abdullah Muhammad Rajab Abd al-Rahman, was reportedly released from custody by Iran in 2015 as part of a prisoner swap.
Last year, Abu al-Khayr al-Masri was reported to have given his blessing to a decision by al-Qaeda's affiliate in Syria, al-Nusra Front, to sever formal ties with the global jihadist network.
Al-Nusra renamed itself Jabhat Fateh al-Sham and insisted it was no longer linked to an external entity, but the US kept the group on its list of foreign terrorist organisations and continued to target it in air strikes.
In January, Jabhat Fateh al-Sham dissolved itself and formed an alliance with four smaller Syrian jihadist groups called Hayat Tahrir al-Sham. The move seemed another attempt by the group to distance itself from al-Qaeda.
Since then, Tahrir al-Sham has fought other rebel groups for control of Idlib province and implied it was behind Saturday's suicide bomb attacks on military targets in the government-controlled city of Homs.