One of Syria's most powerful military officers has been killed by Islamist rebels in eastern Syria.

Maj. Gen. Jameh Jameh, the head of Syria's military intelligence, was shot dead by a rebel sniper in the city of Deir al-Zour, on the shores of Euphrates River, Syrian activist said.

"Jameh was killed by a sniper shot in the Reshdiya neighbourhood of Deir Ezzour during clashes there," said the Syrian Observatory for Human Rights (SOHR).

Deir al-Zour lies in the name sake oil-rich province, which is largely controlled by rebel groups including al-Qaida linked Jabhat al-Nusra.

SOHR said Jameh was killed during fighting between government troops and rebel fighters from al-Nusra and another group named Liwa al-Fatihun min Ard al-Sham.

Syria state-run TV confirmed the death saying Jameh died "pursuing terrorists" to defend Syria and its people.

Jameh is the most senior military officer to be killed since four top officials, including defence minister General Daoud and the brother-in-law of Syrian President Bashar al-Assad, General Assef Shawkat, died in a bomb attack on a cabinet meeting in Damascus in July 2012.

"The general [Jameh ] was known to be one of the officers close to Bashar al-Assad, and one of the main security officials in Syria," SOHR said.

Jameh had an infamous reputation due to his alleged role in the assassination of former Lebanese Prime Minister Rafik Hariri in 2005.

At the time Syria was withdrawing its troops from Lebanon at the end of a three-decade military presence and still held an extensive influence in Beirut.

Jameh was the commander of Syrian Military Intelligence's headquarters in the Lebanese capital when Hariri's motorcade was blown up by a massive explosion that killed a total of 22 people.

Jameh was questioned by investigators as part of a UN inquiry into the killig.

Five members of Lebanese Shia Islamist group Hezbollah - which is now fighting alongside Assad's troops in Syria - have subsequently been indicted.

"It is difficult to believe that Hezbollah carried out such an operation without full coordination with Syrian intelligence," said Mustafa Alloush, a senior member in Hariri's Future Movement.

Syria denies any involvement.