BEIRUT: Forces loyal to Syrian President Bashar Assad unleashed on Saturday their heaviest artillery and rocket barrage yet in a week-long battle to dislodge rebels from a strategic western town, activists said.

Pro-Assad troops, including fighters from the Lebanese militia Hezbollah, have been trying to push rebels out of Qusair. They have gained ground, but rebels have clung to some positions.

Qusair is important to Assad because it sits on a land corridor linking two of his strongholds, the capital of Damascus and towns on the Mediterranean coast. For the rebels, holding Qusair means protecting a supply line to Lebanon 10 kilometres away.

Saturday's barrage of rockets and tank shells began after daybreak, said Qusair activist Hadi Abdullah and the pro-opposition Syrian Observatory for Human Rights, which is based in Britain. Both said it was the most intense shelling since the regime launched its offensive there a week ago. They also reported heavy gunfire. There were no immediate reports of casualties.

The fighting over Qusair has highlighted Hezbollah's growing role in Syria's civil war. The militia initially tried to play down its involvement, but could no longer do so after several dozen of its fighters were killed in Qusair and buried in large funerals in Lebanon.

Saturday's push comes ahead of a widely anticipated speech by Hezbollah chief Hassan Nasrallah, his first since the offensive began. The speech is to mark the anniversary of Israel's May 2000 withdrawal from southern Lebanon, commemorated each year by Hezbollah as a major military victory. However, this year's anniversary comes at a time when Hezbollah is facing growing criticism in Lebanon for its involvement in the war in neighbouring Syria. Hezbollah is also facing repercussions in Europe over its support for the Syrian military.

Earlier this week, France and Germany joined a push by Britain to have the EU declare Hezbollah's military wing a terrorist organisation. Such a move, long sought by the US, would hamper Hezbollah operations in Europe.

Late Friday, Hezbollah's deputy chief, Sheikh Naim Kassem told the Lebanese TV station Al-Mayadeen that the EU would make a ''big mistake,'' but that such warnings don't concern the group. He did not elaborate.

Meanwhile, Syria's fractured political opposition was meeting for a third day in Istanbul, Turkey on Saturday to elect new leaders, try to widen its base and forge a unified position ahead of possible peace talks with the regime.

The US and Russia want to bring together representatives of the opposition and the Syrian government at an international conference in Geneva for talks on a possible transition government. Much remains up in the air, including the date, the agenda and the list of participants.

On Friday, Syria ally Russia said the Assad regime accepted in principle to attend talks in Geneva, though there has been no official statement from Damascus.

The opposition is deeply suspicious about Assad's intention to hold serious peace talks, and senior opposition figures have ruled out attendance unless Assad's departure tops the agenda of such talks.

Louay Safi, a senior member of the Syrian National Coalition, the main opposition bloc, dismissed the statement made by Moscow about Syrian attendance.

''This announcement has to be made by the Syrian government, not the Russians,'' he said Saturday by phone from Istanbul.