ARBIL, Iraq - US Vice President Joe Biden said on Thursday he had no indication the attack on the British embassy in Tehran was orchestrated by Iranian authorities, but it was another example of why the country was a "pariah."

Biden, wrapping up a three-day visit to mark the end of the American war in Iraq, played down the risk of Iran exploiting the departure of US troops by the end of the year.

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He also said the threat of instability in Syria spilling across its borders was not grounds for Syrian President Bashar Assad to stay in power.

"I don't have any indication how and/or if it was orchestrated," Biden told Reuters in an interview, of the attack on the British embassy on Tuesday. "But what I do know is that it is another example to the world and the region that these guys are basically a pariah internationally."

Biden will travel later on Thursday to Turkey and he was expected to discuss the situation in Syria during several days of meetings that will dwell heavily on the implications of the unrest sweeping the Arab world.

A bloody crackdown against pro-democracy demonstrators has raised fears the violence in Syria could spawn a regional sectarian conflict, particularly in neighboring Iraq. Biden said the best thing was for Assad to go.

"It is clear to us Assad is the problem in Syria, and it is not illegitimate for any of Syria's neighbors to wonder what comes next but... the first and most important thing is for him to leave."

Turkey has joined sanctions slapped on Syria by the Arab League and voiced concern the country's escalating violence could spark a pan-regional conflict between Sunni and Shi'tes.

Biden acknowledged there was the potential for the region to take a serious turn for the worst.

"It is an Arab Spring and I guess if you look at a worst case scenario and think the worst, it could become an Arab winter. But I think it is progress."

US troop levels in Iraq are down to around 12,000 and almost all will be gone by the end of the year. Critics say this will create a power vacuum that will allow Iran to increase its influence on Baghdad, but Biden does not agree.

"I think the potential for Iran to influence events in Iraq is vastly overstated," he said.

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