By Dominic Evans
BEIRUT (Reuters) - Syria's opposition and the government of President Bashar al-Assad seem to be preparing to take part in an international peace conference against a background of some of the worst fighting this year.
On Tuesday, Lebanese Hezbollah fighters and Syrian soldiers, backed by air strikes and artillery, renewed an offensive aimed at driving Syrian rebels from the town of Qusair near the Lebanese border, opposition activists said.
Bitter fighting in the town since Sunday has claimed the lives of up to 50 fighters from the Iranian-backed guerrilla group, among their worst losses so far in the war, and prompted renewed fears that Syria's two-year-old civil war will spread beyond the country's borders.
A senior U.S. official said reports from Syrian rebel commanders indicated that Iranians were at Qusair along with fighters from Lebanese Hezbollah.
"It is the most visible effort that we have seen of Hezbollah to engage directly in the fighting in Syria as a foreign force and we understand there are also Iranians up there. That is what the Free Syrian Army commanders are telling us," said the official, who spoke on condition of anonymity.
While it was not clear what the Iranians were doing, he said "they could be doing a little of both advising and fighting".
In further hostilities, Syria announced for the first time since the civil war began that its soldiers had targeted Israel's armed forces. It said its troops had destroyed an Israeli vehicle that crossed into Syrian territory from the Israeli-occupied Golan Heights.
Israel's most senior general warned Assad he would bear the consequences of any further escalation on the Golan Heights. Israel said the incident took place on its side of the Golan ceasefire line, that the vehicle was only damaged, none of its soldiers were hurt and they returned fire.
Violence has left at least 80,000 Syrians dead as world powers have argued over how to end it but on Tuesday foreign diplomats said Assad's government was preparing to join a peace conference promoted by the United States and Russia, while his opponents said they too were ready to take part.
In the northern Lebanese city of Tripoli, three people were killed in further sectarian fighting on Tuesday, security sources said, raising the death toll from four days of sporadic violence related to Syria's civil war to nine.
The sources said 60 people, including civilians, had been wounded over that period in Tripoli, where an pro-Assad Alawite minority lives on a hill overlooking the mainly Sunni Muslim port city.
With international pressure rising for a return to the table by both sides, the United Nations-Arab League mediator, Lakhdar Brahimi, said the U.N. was working to organize the peace talks.
"The Syrian people are building great hopes on the conference, as the opposition prepares itself to take part and likewise the Syrian regime prepares to take part in this conference," he told reporters at the Arab League in Cairo.
Western powers and Russia back opposing sides in the cross-border Syrian conflict - though Washington and its allies have been unwilling to intervene militarily in support of fractured and partly Islamist rebel forces.
The talks are due to take place in Geneva in June. U.S. Secretary of State John Kerry is due to discuss current planning for the conference at a meeting in Jordan on Wednesday of the "Friends of Syria" club of countries, many of which are skeptical about the peace initiative.
Brahimi said: "There are many problems in the preparation for this conference, the first of which is the formation of the delegations of the regime and the opposition."
Syria's opposition is also due to meet in Istanbul on Thursday to announce its stance while the Arab League's Syria committee will meet in Cairo.
European Union diplomatic sources said Assad had put forward the names of five officials from his administration to take part in the talks.
The list includes Prime Minister Wael al-Halki and more junior officials. According one EU diplomat, the opposition has rejected some of the names because of their lack of influence.
But opposition figures say they are likely to agree to attend the talks anyway in a bid to isolate Assad.
Diplomats say it is too early to say when the talks might take place.
The senior U.S. official hinted at U.S. skepticism about Russia's commitment to a negotiated solution, suggesting that Russian arms deliveries to the Assad government would probably stiffen the president's resolve to hang on to power.
"The Russians, on the one hand, strongly support, they say, holding this conference in Geneva but on the other hand they are continuing to deliver weapons, very modern weapons, very dangerous weapons to the Syrian regime," said the U.S. official.
Assad is battling an uprising in which rebel forces, including radical Islamists, have taken swathes of territory.
The latest battleground is Qusair, which lies on an important supply route from Lebanon for rebels in the central province of Homs.
Opposition activists said fighting was raging in the town, as well as in areas to the east where several army bases are located, and in the Hezbollah-held southern and western approaches to the town.
The number of rebel and civilian deaths in the last 48 hours had reached more than 100, according to the opposition.
Syrian state media have said Assad's forces had reasserted control over most of Qusair, but rebels said the offensive had been ineffectual.
The senior U.S. official said there were concerns that if Syrian government forces captured Qusair, there would be retaliation against the thousands of civilians there.
"And so it is important that the regime know that the world is watching this and we are watching this and we will know if they commit massacres and we will know who it was that committed it and they will be held accountable," the U.S. official said.
Assad, who is from Syria's minority Alawite sect and has long backed Hezbollah in Lebanon, is fighting a Sunni-led revolt which began with peaceful protests in March 2011. His violent response eventually prompted rebels to take up arms.
(Additional reporting by Arshad Mohammed in Muscat, John Irish in Paris and Ayman Samir in Cairo; Writing by Giles Elgood; Editing by Alastair Macdonald)