The White House said Wednesday that Syrian President Bashar al-Assad was "losing control'' of his country following a suicide bombing in Damascus that killed three top security chiefs.

"It's clear that the Assad regime is losing control of Syria,'' National Security Council spokesman Tommy Vietor said, stressing the urgent need for a political transition to avoid a "lengthy and bloody sectarian civil war."

Iranian Foreign Minister Ali Akbar Salehi condemned a suicide bombing which killed three Syrian security chiefs on Wednesday, in a telephone call with his counterpart in Damascus.

"Salehi condemned the terrorist action committed today in Damascus during a telephone conversation with Walid Muallem,'' the official IRNA news agency reported.

Tehran is the main regional ally of Syrian President Bashar al-Assad's regime, advocating dialogue between the government and opposition to end a 16-month uprising that has killed thousands of people.

According to IRNA, Salehi called for "an immediate end... to foreign interference and arms shipments... to Syria and the support of some regional and international parties for terrorist action.''

"The Islamic Republic of Iran condemns all violence and destructive action and believes that dialogue is the only solution to end the Syrian crisis,'' said the minister.

Iran regularly accuses some Western and Arab states of supporting rebels inside Syria, including by sending them weapons.