Beirut (AFP) - Fighters from Lebanon's Hezbollah have killed a top jihadist from the Islamic State group in Syria who allegedly planned bloody attacks in Lebanon, a monitoring group said on Tuesday.

The Syrian Observatory for Human Rights said Hezbollah fighters had killed Abu Abdullah al-Iraqi in the Qalamun region in Syria, near the border with Lebanon.

He "was one of the officials in the Islamic State in charge of preparing suicide attacks", the group said.

"He was killed by a roadside bomb planted by Hezbollah that detonated as his vehicle passed by," the NGO added, saying three other jihadists were also killed in the blast.

Hezbollah's Al-Manar television station meanwhile reported Iraqi's death, but said he had been killed by the Syrian army.

"The Islamic State official Abu Abdullah al-Iraqi has been killed in a Syrian army operation in Qalamun," al-Manar said.

"He was in charge of preparing suicide bombers and the cars used in bomb attacks including those carried out in Lebanon," the station said, broadcasting a photo said to be of Iraqi's bloodied head.

Lebanon's Shiite Hezbollah movement is a close ally of the Syrian regime and has been fighting alongside government troops against an uprising there for more than a year.

Its involvement has helped the army to recapture key territory, but drawn the ire of many in Lebanon, where Hezbollah strongholds have been targeted multiple times by suicide and car bomb attacks.

The attacks have been claimed by jihadist groups who threatened to continue so long as Hezbollah remains in Syria.

In mid-April, the army and Hezbollah took control of most of the Qalamun region, but pockets of opposition fighters, including jihadists, remain in the mountainous border area there between Syria and Lebanon.