At least one member of Lebanon's Hezbollah group has been killed overnight in clashes with Syrian rebels in Lebanon's eastern border region Al Jazeera reported.
Lebanese security sources reported the death on Sunday, saying that about 15 rebels had also been killed in the violence, east of Baalbek in the Bekaa Valley.
The conflict in Syria has been increasingly spilling over to its smaller neighbour, with deadly fighting shaking the northern Lebanese city of Tripoli, and rockets hitting the Bekaa Valley and Beirut's southern suburbs.
Al Jazeera's Mysa Khalaf, reporting from Tripoli, said that about 17 had been wounded in clashes in the Bab Tabbaneh neighbourhood, and that residents were trying to remove the wounded from the area.
Syrian rebels have said they will carry out attacks inside Lebanon in response to Hezbollah's support for President Bashar al-Assad's assault on Qusayr, a strategic town for rebel weapons supplies and fighters coming into Syria from Lebanon.
Al Jazeera's Rula Amin, reporting from Beirut, said there were reports that the latest clash started when Hezbollah fighters came across rebels setting up rocket launchers close to Baalbek.
"It is the first time we have seen a face-to-face clash between rebels and Hezbollah fighters inside Lebanon," our correspondent said.
"There have been rocket attacks on Hezbollah strongholds in that area before, and so it is not a surprising development that rebels are trying to target this town.
"It is significant that there has been a clash inside Lebanese territory as it shows how determined the Syrian rebels are to target Hezbollah strongholds. It also shows how vulnerable Lebanon has become that they were able to cross the border."
Meanwhile, the Syrian government has responded to calls from international aid organisations for civilians trapped in the flashpoint Syrian city of Qusayr to be evacuated, as rebel fighters faced a fresh assault.
Syrian state TV reported that the foreign minister has said the International Committee of the Red Cross (ICRC) will be allowed into Qusayr "as soon as military operations are over".
'Grave concern'
UN agencies and the ICRC appealed to both sides in the fighting on Saturday to let the civilians, including an estimated 1,500 wounded, leave Qusayr.
Britain circulated a draft declaration at the UN Security Council, voicing "grave concern about the situation in Qusayr".
Russia, however, blocked the draft because the UN had failed to speak out when Qusayr was seized by rebels more than a year ago.
The government's military campaign on Qusayr started two weeks ago, in an attempt to regain control of the strategic city bordering Lebanon. It is believed that Hezbollah has sent an estimated 1,700 fighters to support the government's assault.
Syrian forces have recently captured the northern district of Arjun in Qusayr, leaving rebels little chance to escape.
Activists said that escape routes for civilians have become unsafe. They reported this week that a a convoy of civilians seeking to flee Qusayr was attacked by Syrian forces.
Ceasefire needed
Valerie Amos, UN emergency relief coordinator, and Navi Pillay, UN high commissioner for human rights, have said there is "an urgent need of immediate evacuation for emergency medical treatment".
But a humanitarian corridor could only be created if both sides agreed, Rupert Colville, Pillay's spokesperson, told Al Jazeera.
"There should be a ceasefire at least and they let the civilians and the wounded get out and also let some aid in as well. Civilians who stay behind will need food and water," he said.
The control of Qusayr is essential for the rebels as it is their principal transit point for weapons and fighters from across the border in Lebanon.
It is also strategic for the government because it is located on the road linking Damascus with the Mediterranean coast, its rear base.