The United States yesterday said Iran is stepping up support for the Syrian regime and that Russia is still arming it, heightening fears the conflict may spill over the country’s borders.

The assessment by US Secretary of State Hillary Clinton came as US Vice President Joe Biden prepared to discuss the crisis with Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib. Fresh concerns about the 22-month conflict drawing in the wider region arose after Damascus threatened to retaliate over a reported Israeli air raid on a research facility and close ally Iran warned the attackwouldhave "graveconsequences".

Israel has frequendy warned that if Syria's chemical weapons fall into the hands of archfoe and close Damascus ally, Lebanon's Shia movement Hezbollah, this would be a casus belli.

Hillary gave a grim assessment of the conflict, warning of "the dangers of an increasing civil war and a potential proxy war".

"The worst kind of predictions of what could happen, both internally and spilling over the borders of Syria, are certainly within the realm of the possible now," she said. Iranians "have been actively involved (in Syria) from the very beginning. It appears that they may be increasing that involvement and that is a matter of concern to us."

She noted Russia was propping up the regime despite US efforts to work for an international solution to a conflict the UN says has cost more than 60,000 lives.