* Shi'ite Hezbollah and Sunni Nusra Front suffer casualties
* Syrian war has become increasingly sectarian
BEIRUT, July 17 (Reuters) - Hezbollah and the Syrian branch of al Qaeda have fought a deadly five-day battle in Syria near the border with Lebanon, a Hezbollah source and a fighter for the Nusra Front said on Thursday.
Hezbollah is a Shi'ite Muslim militant group that has fought alongside troops loyal to President Bashar al-Assad in Syria's civil war, now in its fourth year. Al Qaeda's Nusra Front is a Sunni militia that considers all Shi'ites heretics deserving death.
A Nusra Front member said Hezbollah had lost many fighters over the past few days during the clashes in Syria's Qalamoun mountains. On Wednesday, Hezbollah tried to enter Syria from two Shi'ite Lebanese villages on the border when they were ambushed by Nusra Front fighters.
"We saw them trying to sneak in," he said. "We waited for them, and when they arrived and before they were able to load their guns or rocket-propelled grenades, we attacked. Some of them escaped."
A Hezbollah source confirmed there had been an attack and said the group had lost three fighters during the Wednesday assault, upping the death toll to nine Hezbollah fighters killed in the area this week.
The Nusra Front fighter said his units had lost "martyrs" but did not give a figure. A Lebanese security source said 26 Nusra Front fighters had been killed this week in the area of the fighting, 3 km (2 miles) from the Lebanese town of Arsal.
Syria's war has been the battleground for a struggle between groups supported by Sunni Muslim states including Saudi Arabia and Qatar, and Assad's government which is backed by Shi'ite Iran.
The conflict began with pro-democracy protests in 2011 and rapidly descended into sectarian civil war in which more than 170,000 people have been killed. According to the United Nations, 10.8 million Syrians now urgently need aid. (Reporting by Mariam Karouny; Writing by Oliver Holmes; Editing by Andrew Roche)