(CNN) — Secretary of State John Kerry sharply criticized Syrian President Bashar al-Assad on Monday, accusing him of soliciting help from Iran and its fighters on the ground in Syria as well as fighters from Hezbollah.
"A designated terrorist organization has now crossed over from Lebanon into Syria and is actively engaged in the fighting," Kerry said at a State Department press availability about the Syrian civil war.
Al-Assad, he said, is willing to use Scud missiles against civilians and noted an "extraordinary number" of people are trapped in the besieged rebel stronghold of Qusair near the Lebanese border.
"He will not allow humanitarian aid to go in until the military has finished what (it) intends to do," Kerry said.
"So I think the world is seeing the actions of a person who has lost touch with any reality except his own and is willing to wreak any kind of punishment on his own -- the people of his country -- simply so that he can maintain power," he said.
"This situation is obviously intolerable, unacceptable by any standard and I think all civilized countries would call on the Assad regime to pull back from that and to engage in the peace process," Kerry said.
Kerry described as a "very difficult process" a conference that so far has failed to bring the opposition and the government together for peace and transition talks.
"We are trying to prevent the sectarian violence from dragging Syria down into a complete and total implosion where it has broken up into enclaves and the institutions of the state have been destroyed, with God knows how many additional refugees and how many innocent people killed," he said.
Kerry said he spoke with his Russian counterpart, Sergey Lavrov, on Friday and that Lavrov is "deeply committed" to supporting the conference, but added the situation in Qusair and the possibility of Moscow supplying its Syrian ally with air defense missiles "could put this kind of thing at risk."
The United States continues to lay the ground for the possibility of a negotiated solution to the violence in Syria and said Under Secretary of State Wendy Sherman will travel to Geneva where she will meet with Russian officials.