* Capturing Yabroud would sever cross-border rebel supply line
* Government regaining ground, initiative in 3-year conflict
* Opposition leader renews plea for arms from Western leaders
* EU's Ashton urges Syria to meet chemical weapons deadline
By Stephen Kalin
BEIRUT, March 15 (Reuters) - The Syrian army entered eastern districts of the town of Yabroud, the last rebel bastion near the Lebanese border north of Damascus, on Saturday and tightened its grip on the remaining rebels there from the al Qaeda-linked Nusra Front.
Soldiers advanced inside Yabroud and "eliminated terrorist strongholds", said a reporter on Syrian state television SANA who was broadcasting live from the town's outskirts.
He said the army had taken control of hills and mountaintops southeast of Yabroud, gaining a strategically advantageous position.
A military source confirmed to Reuters that the army had taken a series of peaks and said it had "fastened pincers around Yabroud."
Capturing the town would help President Bashar al-Assad cut off a cross-border rebel supply line from Lebanon. It is near the highway linking Damascus to the former commercial hub of Aleppo in the north and to the Mediterranean coast in the west, where Assad's minority Alawite community is concentrated.
The government has been making incremental gains along the highway as well as around Damascus and Aleppo in recent months, regaining the initiative in a conflict which enters its fourth year this month.
Thousands of people fled Yabroud, a town of about 40,000 to 50,000 people roughly 60 km (40 miles) north of Damascus, and the surrounding areas after it was bombed and shelled last month ahead of the government offensive.
The Syrian Observatory for Human Rights monitoring group said the government pounded some districts of Yabroud with barrel bombs on Saturday and shelled the town's outskirts.
The SANA reporter said the army had imposed a cordon around the western part of the town before entering from the east. "With the entrance of the Syrian army forces from the east and the west, the noose was tightened around the (armed) groups," he said.
Some of the rebels had withdrawn to Rankos, a village 30 km (20 miles) southwest of Yabroud, said SANA.
But others, mainly from the Nusra Front, had stayed in Yabroud and were fighting the advancing government forces. The reporter said the Nusra militants had blocked roads to prevent other rebels from retreating.
"Most of them are non-Syrian nationals and therefore are still insistent on remaining in Yabroud even though the Syrian army is dealing strong, severe strikes against these groups."
AL QAEDA-LINKED COMMANDER KILLED
Footage broadcast by Al Mayadeen television from the outskirts of Yabroud and some of its eastern districts showed soldiers charging through a field towards an arched gate and a sign saying "Welcome to Yabroud".
Soldiers fired machine guns and advanced through what appeared to be lightly populated residential areas. Gunfire could be heard in the background.
Other footage from Al Mayadeen and Hezbollah-run Al Manar television showed dark plumes of smoke rising over a section of the town, as the army targeted suspected rebel holdouts.
The Observatory, an anti-Assad group, said five rebel fighters were killed in heavy fighting between government forces supported by Lebanese Shi'ite militant group Hezbollah and rebel factions including the Nusra Front, al Qaeda's official branch in Syria, and the Islamic State of Iraq and the Levant, an al Qaeda splinter group.
A senior commander in the Nusra Front was killed late on Friday on the outskirts of Yabroud during shelling and clashes with the army and Hezbollah fighters, the Observatory said.
Abu Azzam al-Kuwaiti was the deputy leader of the Nusra Front in Qalamoun, the mountainous zone between Damascus and the Lebanese border where Yabroud is situated.
He had been a principal negotiator in the prisoner exchange last week which secured the release of 13 Greek Orthodox nuns held by the Nusra Front since December, according to the Observatory.
As rebels appeared on the verge of losing another town, opposition leader Ahmad Jarba urged world leaders to fulfill their promises to provide weapons to the Free Syrian Army.
"I remind them that the time they are trying to buy today through procrastination will tomorrow put a knife to the neck of the region and peace and security in the world," he said in Istanbul.
Jarba was speaking to mark the third anniversary of the conflict, which began with peaceful protests against Assad but turned into an armed insurgency after a violent security crackdown on demonstrators. More than 140,000 people have been killed and 2.5 million have fled abroad as refugees in an increasingly sectarian civil war.
European Union foreign affairs chief Catherine Ashton, in a statement on Saturday to mark the anniversary, urged Assad's government to comply with a June 30 U.N. deadline to eliminate its entire chemical weapons programme.
Damascus did not meet a commitment to destroy 12 chemical production facilities by March 15, after having already missed several deadlines laid out in last year's agreement.
Ashton also said the international community had a responsibility to bring the conflict to an end.
"The tragedy in Syria has no parallel in recent history. The only solution to the crisis is a political one: the establishment of a transitional governing body, and a genuine Syrian-led inclusive political process to establish a democratic and pluralistic Syria."