The UN is calling for more clarity from Damascus on the reasons behind the sudden stationing of thousands of troops on the Lebanese border, a move that has raised anxieties in Beirut.

Michael Williams, who takes up his new post next month as the UN’s special coordinator for Lebanon, played down concerns over renewed Syrian military intervention in Lebanon and said there was a case for moving troops to the border to stem the flow of smuggling.

But he told the Financial Times: “We need clarity on the purpose of the 10,000 troops on the border.” He said he understood the worry of some Lebanese, but did not believe that the troop movement was “as serious as some of the concerns expressed.”

Damascus’ decision has upset Lebanon’s pro-western parliamentary majority, three years after Damascus was forced to pull its soldiers out and end nearly three decades of domination over its smaller neighbours.

The move, which Syria said was aimed at curbing smuggling, was also surprising because it came at a time when the two countries have taken steps to ease tensions. Damascus has agreed to open diplomatic relations with Lebanon, and an exchange of embassies is planned for the first time.

Damascus’ allies in Beirut – led by Hizbollah, the Shia militant group – have consolidated their political power in recent months, winning a blocking minority in a government of national unity. They have also been holding reconciliation talks with pro-western leaders of the parliamentary majority.

The reconciliation efforts are part of a May agreement reached between feuding Lebanese leaders in the Qatari capital of Doha. The peace deal, brokered by Arab states, was struck after Hizbollah and its allies briefly took over parts of west Beirut in the worse fighting since the end of the civil war in 1990.

Mr Williams said the anxiety over the Syrian troop movement underlined the fragility in the country despite the recent reconciliation. “The more reconciliation signs we see, the better,” he said.

The UK diplomat, who has been serving as Britain’s Middle East envoy, is charged primarily with the implementation of UN resolution 1701, which helped end the month-long war between Hizbollah and Israel in 2006.

Although the Israel-Lebanon border has been quiet for the past two years – partly thanks to thousands of UN troops now stationed in southern Lebanon – both Hizbollah and Israel have repeatedly warned of a renewal of conflict.

Mr Williams said it was “remarkable” that not a single shot has been fired since the cessation of hostilities, particularly when there is no formal ceasefire in place. Hizbollah and Israel also agreed an exchange of prisoners this summer, a move that seemed impossible two years ago.

The tensions between the two sides, however, have led to a different kind of war: Hizbollah accuses Israel of the assassination of a top commander, Imad Moughniyah, early this year, and has vowed revenge. In August General Mohammed Suleiman, a senior military advisor to Bahsar al-Assad who was also believed to be the government’s liaison with Hizbollah, was also mysteriously killed. Syria has not confirmed the assassination, however, nor accused anyone.

Israel, meanwhile, says Hizbollah has been seeking to retaliate for the Moughniyeh killing by hitting Israeli targets outside the country

Mr Williams said a renewal of border conflict between Lebanon and Israel was still unlikely over the next 12 months, despite the rhetoric on both sides. “I hope I can look forward to another two years of calm,” he said.

He is also hoping that in the next few weeks, Israel and Lebanon will agree to resolve the dispute over the border village of Ghajar, with the UN taking over the northern part before it reverts back to Lebanon.

Solving Ghajar, said Mr Williams, could “offer perhaps some hope” of a resolution of the thornier issue of Shebaa Farms.

This small strip of land is claimed by Lebanon and occupied by Israel, which says it belongs to Syria, rather than Lebanon. When Israel withdrew from southern Lebanon in 2000, the UN maintained that Shebaa indeed belonged to Syria.

Since then, however, the UN has commissioned a study that found new maps, title deeds of land and taxation records. Although the issue has been raised with Israel, little progress has been achieved so far. Help from Syria on documents has not been forthcoming either.

A solution to Shebaa Farms, in turn, would help Lebanon address the disarmament of Hizbollah – which is required by UN resolutions but increasingly seen as unlikely to be achieved.

The group maintains that its weapons are aimed at protecting Lebanon and winning back occupied territory, although in recent weeks some Hizbollah officials have said they will hold on to their weapons even if Israel withdrew from Shebaa.

Disarmament is to be discussed during the recently launched national dialogue under the leadership of the new president and former army chief, Michel Suleiman, and within the context of developing a national defence strategy for the country.

But given the power that Hizbollah wields in Lebanon, it is unlikely that the dialogue will lead to any significant shift in its attitude.

Mr Williams acknowledged that progress on disarmament will be slow. He argued that full disarmament is always difficult in such situations, pointing in particular to the Northern Ireland case.

The “regional context,” he said, was also not favourable, in reference to the wider struggle in the region between the US and its Arab allies on one hand and Syria and Iran on the other.

“There is a genuine discourse that needs to be addressed – what is the appropriate defence posture for Lebanon,” said Mr Williams. “I have some sympathy for the view that Lebanon’s armed forces are not strong at the moment.”

He said that although the army, which was until recently led by Mr Suleiman, is one of the “great success” stories in Lebanon – it kept the country together despite the political rifts and defeated a militant group that had taken over a Palestinian refugee camp in the north of the country last year – it still lacked equipment, and was behind other armies in the Arab world.

“Does Lebanon have the appropriate strength, equipment to address its defence needs?” asked Mr Williams. Disarmament, he added, can be handled in different ways, citing cases in Africa where former rebels were eventually integrated into national army.

Mr Williams said he would offer the help of the UN, given its wealth of experience in dealing with similar situations. It was up to the president, he said, to determine whether he wanted the UN to have a formal role, but expected that any assistance would come at a later stage and would be of a technical advisory nature.

At a time when Lebanon is supposed to be discussing the disarmament of Hizbollah, the group has made no secret that it has replenished its arsenal of rockets since the 2006 war. And this has led some other groups in Lebanon who see Hizbollah as a threat to also start buying weapons too.

Mr Williams said the rearmament of the Shia group was “a big concern” that had been raised with both Hizbollah and Syria. Talk of others also arming was also worrying.

“One hears rumours and stories about arms, it’s a real worry,” he said. “The future for the Lebanese people and the state must be to take arms out of politics for the monopoly to rest with the armed forces.”