The United States warned Friday that Syria's allies Iran and Hezbollah could be planning attacks in Europe, as Washington boosted sanctions against the beleaguered Damascus regime.
The State Department and US Treasury unveiled fresh measures in response to the mounting conflict in Syria, where 21,000 people have been killed in the violence of the past 17 months in the face of a diplomatic stalemate.
But a senior US security official warned the Western pressure might not succeed without cost, with Iran and the Lebanese militant group Hezbollah allegedly plotting revenge attacks on Western or Israeli targets.
"Our assessment is that Hezbollah and Iran will both continue to maintain a heightened level of terrorist activity in operations in the near future," said Daniel Benjamin, the US State Department's counter-terrorism coordinator.
"We are increasingly concerned about Hezbollah's activities on a number of fronts, including its stepped up terrorist campaign around the world," he said.
"And we assess that Hezbollah could attack in Europe or elsewhere at any time with little or no warning," he said in a conference call with reporters to announce that Syria's oil firm Syrtol had been placed under sanctions.
Hezbollah is backed by Damascus and Tehran and has been accused -- so far without proof -- both of playing a role in recent bomb attacks on Israeli civilian targets in Europe and Asia and of backing Assad's forces.
"Hezbollah's extensive support to the Syrian government's violent suppression of the Syrian people exposes the true nature of this terrorist organization," said David Cohen, a senior US Treasury official.
Iran, Bashar al-Assad's authoritarian Syrian regime and Hezbollah, an armed Lebanese faction, dub themselves the "axis of resistance" against Israeli and US ambitions in the Middle East.
But today it is they who are under pressure, with Assad's regime facing a determined armed rebellion -- despite a brutal crackdown against his opponents -- and all three entities facing international economic sanctions.
Russia and China have blocked the UN Security Council from taking action, but the United States and it allies have been gradually escalating their own sanctions regimes and stepping up support for the Syrian rebels.
On Friday, the US administration announced that it was adding Hezbollah, already deemed an "international terrorist organization," to the list of entities under sanctions for supporting Assad's regime.
And it said that Syrtol wound be sanctioned for exporting fuel to Iran.
"This kind of trade allows Iran to continue developing its nuclear program while providing the Syrian government with resources to oppress its own people," State Department spokesman Patrick Ventrell said.
Benjamin warned that Hezbollah was poised to step up its violence.
"Hezbollah maintains a presence in Europe and its recent activities demonstrate that it is not constrained by concerns about collateral damage or political fallout that could result from conducting operations there," he said.
"Hezbollah believes there have been sustained Israeli and Western campaigns against the group and its primary backers Iran and Syria over the past several years, and this perception is unlikely to change," he added.
"Both remain determined to exact revenge against Israel and to respond forcefully to the Western-led pressure against Iran and Syria," he said.
"This suggests more acts of terrorism by both Hezbollah and Iran are likely and they will continue to pose a serious threat for the foreseeable future.
Israel blames Hezbollah for plotting attacks in more than 20 countries over the past two years, including last month's bomb blast on a bus in the Bulgarian resort of Burgas that left five Israeli tourists and the local driver dead.
European security services have said there is not yet enough evidence to indict Hezbollah for the attack, but that they are investigating.
For Washington, Benjamin said: "Although the investigation continues and we are not in a position to make a statement about responsibility, the attack does resemble Hezbollah's plotting earlier this year."