The ghost of Abu Mussab al-Zarqawi, the notorious jihadist leader killed in a U.S. airstrike in Iraq in June 2006, pervades the operations of Jabhat al-Nusrah, the Islamist group that spearheads rebels fighting the Damascus regime in Syria.

Increasingly, the expanding and highly effective jihadist group is seen as an outgrowth of Zarqawi's al-Qaida in Iraq that's killing scores of people every month in an offensive against the government of Prime Minister Nouri al-Maliki.

Jabhat Al-Nusrah's operations are causing growing alarm not only within the embattled regime of Syrian President Bashar Assad, whose forces and allies are bearing the brunt of the group's attacks, but in neighboring Jordan, Lebanon and Turkey where jihadist groups are reported to be gaining strength.

There are also fears that Jabhat al-Nusrah could have connections with jihadist cells in Europe that were linked to an organization set up by Zarqawi in 2001 called Ansar al-Islam, or Partisans of Islam.

In 2004, Western intelligence officials said Ansar al-Islam was involved in bombings in Casablanca, Madrid and Istanbul in 2003-04 and was linked to al-Qaida in Iraq.

Zarqawi, whose real name was Ahmed Fadil al-Khalayleh, set up an extremist Sunni group called al-Tawhid w'al-Jihad, or Unity and Holy Struggle, in the 1990s dedicated to overthrowing the Hashemite monarchy in Jordan.

He organized and led AQI until he was killed June 7, 2006, when a pair of F-16s dropped two 500-pound laser-guided bombs on his hideout near Baqubah, north of Baghdad.

Whether Zarqawi's extensive operational capabilities, mapped by Western intelligence officials, extended as widely as suggested in 2004 remains open to question. But more than 100 members of the loose network of groups he allegedly brought together from 2001-06 were arrested in Europe since 2001.

Testimony of several activists captured in that period suggested that Ansar al-Islam and al-Tawhid had established a widespread cell network across Italy, Germany, the Netherlands, Spain, Belgium, Norway and Britain.

One activist, a 30-year-old Algerian named Abderrazak Mahdjoub arrested Nov. 23, 2003, in Hamburg, Germany, allegedly provided a wealth of intelligence on the European network.

Al-Nusrah's exploits, including more than 50 suicide bombings against key regime targets, are energizing jihadist groups across the Middle East.

This is being intensified by Ayman al-Zawahiri of Egypt, al-Qaida Central's leader since Osama bin Laden was assassinated in Pakistan by U.S. Special Forces May 2, 2011.

If the Assad regime in Syria is toppled, al-Nusrah will demand a significant, if not dominant, position in a successor regime in which Sunni Islamists will have considerable influence. This would give the jihadists a solid foothold in Syria.

The Lebanese have reported an uptick in jihadist activity.

Jordanian intelligence has thwarted at least one major terrorist plot attributed to al-Nusrah. Israel reports heightened jihadist activity in the Gaza Strip and Egypt's Sinai Peninsula.

The Jordanians have also arrested several jihadists, including two of Zarqawi's cousins, Zayed Sweiti and Firas al-Khalaylah, after they'd spent five months fighting in Syria.

Al-Jazeera reported in December that Zarqawi's brother-in-law, Iyad al-Tubasi, was killed fighting in southwestern Syria near the Jordanian border. It said the Jordanian jihadist who replaced him as the commander of an al-Nusrah unit, Mustafa Abdel Latif Salih, aka Abu Anas al-Sahhaba, had served with Zarqawi in Iraq.

Another brother-in-law, Sami al-Daini, was reportedly appointed to lead an AQI group in Iraq's Diyala province in 2010.

The Quilliam Foundation, a counter-terrorism think tank in London that tracks jihadist groups, said in a report issued Tuesday that al-Nusrah has some 5,000 hardcore fighters, plus several thousand auxiliaries and independent fighters. Syrian opposition sources put its strength as high 10,000.

Quilliam identified al-Nusrah's leader as a Syrian jihadist who goes by the nom de guerre Abu Mohammed al-Golani, believed to be a veteran of Zarqawi's operations in Iraq against the Americans after the 2003 invasion.

His true name isn't known but his nom de guerre indicates his family is from the Golan Heights, a volcanic plateau in Syria that was partially conquered by Israel in the 1967 Middle East war.

"By investigating Zarqawi's old network and using a process of elimination, we have narrowed down possible candidates to one individual," the report said.

"This man is mysterious, having been reported killed twice, in Iraq in 2006 and Syria in 2008."