By Kim Ghattas
BBC State Department correspondent, Washington
Hezbollah is fiercely opposed to Israel
The White House has said it is increasingly concerned over reports that Syria is sending sophisticated weaponry to Hezbollah in Lebanon, a day after the Israeli President Shimon Peres accused Damascus of supplying Scud missiles to the militant group.
The reports are a blow to the Obama administration's attempt to engage positively with the Syrian government of President Bashar Assad after years of tension between Washington and Damascus.
"We are obviously increasingly concerned about the sophisticated weaponry that ... is allegedly being transferred," White House spokesman Robert Gibbs told reporters. "We have expressed our concerns to [the] governments [of Lebanon and Syria]."
Mr Gibbs was responding to questions about the Israeli president's comments. He did not confirm that the transfer had taken place but warned it could have a potentially destabilizing effect on the region.
Hezbollah is backed by Iran; both are Israel's arch enemies. If the militant group obtains medium and long range ballistic missiles, such as Scuds, this could alter the military balance in the region.
Hezbollah and Israel fought a war in 2006 during which Hezbollah fired rockets into northern Israel but Scuds would put all of Israel within reach.
Israel's president has said Syria is supplying Scuds to Hezbollah
Some military experts however believe Hezbollah already has longer-range missiles, in which case this latest development would be important because of the signal it sends at a time of simmering tension in the region.
Israel is worried about Iran's nuclear programme and has often warned it could strike Iran if it felt threatened.
Syria and Hezbollah would be likely to be immediately drawn into the conflict. But Iran's allies could also choose to strike pre-emptively at Israel.
"If [Scuds have been transferred into Lebanon], and we continue to analyze this issue ... clearly it potentially puts Lebanon at significant risk," said State Department spokesman PJ Crowley.
In Washington, the Syrian Embassy dismissed the allegations and accused Israel of trying to divert attention from questions about Israel's nuclear programme. Israel denies it has nuclear weapons.
Sign to Tehran?
Speaking anonymously because of the sensitivity of the issue, a senior US official recently told the BBC he believed Syria had sent new weaponry to Hezbollah.
He said it was more sophisticated than anything the group had had before but would not comment on the specific kind of missiles involved.
When asked what Syria's intentions were, the official said it was unclear.
There is speculation that the flow of weapons into Lebanon is Syria's way of showing Iran it is still a reliable ally.
The US's ability to defuse the crisis depends on where the Scuds actually are - on the border, still in Syria or in Lebanon
The Obama administration has recently been trying to engage diplomatically with President Bashar Assad and had just appointed a new ambassador to Damascus but his nomination is on hold in the Senate.
Ties between Syria and the US had been tense since the 2005 killing of Rafik Hariri, a former Lebanese prime minister.
His assassination was blamed on Damascus which denied any involvement but Washington withdrew its ambassador shortly after.
At that time, the Bush administration also accused Syria of supporting the flow of fighters into Iraq.
The new US administration has been trying to engage with Syria, hoping to peel it away from Iran. Washington is also pursuing both engagement and pressure with Tehran hoping to dissuade it from pursuing a nuclear programme.
But Bashar Assad recently rebuffed Washington when he hosted Iran's President Mahmoud Ahmadinejad and Hezbollah's leader Hassan Nasrallah for lunch in Damascus.
John Kerry recently met President Bashar Assad in Damascas
Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy said the Scud episode was "an unexpected escalation".
Mr Tabler, the author of a forthcoming book on Syria and Bashar Assad, said that the recent developments were not a positive sign for engagement.
Another senior US official told the BBC that the Syrians had been "feeling cocky" recently.
Syria was shunned by the West and some Arab countries, in the aftermath of the Hariri killing.
But two years ago France's President Nicolas Sarkozy re-engaged with Syria, followed by Saudi Arabia and then finally the Obama administration.
It prompted a slew of triumphal editorials in the state-controlled Syrian press, which in essence claimed that Syria had been proven right and that its steadfastness had paid off.
Washington continued to make clear that it had demands it expected Syria to meet, such as stopping the flow of arms to Hezbollah and improving relations with Lebanon.
But critics of the engagement policy, particularly Republicans, said engagement had been an underserved reward.
The second senior US official said part of the reasoning behind sending an ambassador to Damascus was to maintain close contact with the government and in essence talk sense to them at a time when "there was a real danger in the region with combustible circumstances".
It is unclear how Washington will now choose to address the issue of the Scuds with Syria and manage to defuse the crisis.
Senator John Kerry, who heads the Senate's foreign relations committee, was recently in Damascus.
An aide to Mr Kerry said he had been well briefed on intelligence matters before heading to Syria, suggesting the issue had come up in the meeting.
The US's ability to defuse the crisis depends on where the Scuds actually are - on the border, still in Syria or in Lebanon.
"If the Scuds are in Lebanon, it's difficult to see how the Syrians would bring them back," said Mr Tabler.