DAMASCUS (AFP) – President Bashar al-Assad will have to step down as part of a political solution to the Syrian conflict, US Secretary of State John Kerry said on Thursday, echoing a demand from the opposition.

His call came after a US-Russian joint initiative was announced on Tuesday to bring the two sides in the conflict to the table on the basis of an international accord last year that made no mention of Assad’s future.

Israel, meanwhile, reportedly warned Washington of Russian plans to sell Syria sophisticated missile systems.

Speaking in Rome, Kerry said all sides were working to “effect a transition government by mutual consent of both sides, which clearly means that in our judgement President Assad will not be a component of that transitional government.”

His comments were significantly more direct than those he made in Moscow, where the joint US-Russian effort was announced.

Then, while saying it was “impossible for me as an individual” to imagine Assad continuing to govern, Kerry said the decision could only by made by the Syrian people.

In Rome, Kerry also officially unveiled $100 million (76 million euros) in additional US humanitarian aid for Syrian refugees, almost half of which will go to help Jordan cope with a tide of people fleeing the 26-month war.

Washington has now pledged some $510 million dollars in humanitarian aid to the Syrian people, and a further $250 million in non-lethal aid to the rebels fighting to oust Assad.

On Tuesday, Kerry and his Russian counterpart Sergei Lavrov said they would seek to convene an international conference to build on a six-point accord agreed in Geneva last year.

The agreement aimed at finding a path towards a transitional government but made no mention of Assad’s departure, which the opposition says is non-negotiable.

In its response to the initiative, the opposition National Coalition reiterated on Wednesday that any political settlement must start with Assad’s ouster.

The Syrian pro-regime daily Al-Watan on Thursday quoted an official as welcoming the effort.

“We are very optimistic about what’s happening, but we are waiting for more details,” the unnamed official told the paper.

“All the capitals of the world will sooner or later come around to Russia’s position because this position is based on international law and the right of people to decide their fate without foreign intervention.”

Assad-allied Russia has sharply disagreed with the United States and other backers of the uprising, a divergence underlined Thursday by reports that Israel has warned Washington of Russian plans to sell sophisticated missile systems to Damascus.

The Wall Street Journal reported that Israel had informed Washington about the imminent sale of Russian S-300 missile batteries, advanced ground-to-air weapons that can take out aircraft or guided missiles.

Moscow remains one of the Syrian regime’s closest allies, and has continued to supply Damascus with weapons throughout the conflict, which has left more than 70,000 people dead since March 2011.

The regime has also relied increasingly on its alliance with Lebanese Shiite group Hezbollah, with a newspaper on Thursday quoting Assad as saying Syria would “give Hezbollah everything” for their loyalty.

The newspaper also quoted Assad as saying his country would follow the militant organisation’s model of “resistance” against Israel, after the Jewish state twice carried out strikes against Syrian military targets last week.

Hezbollah, whose leader Hassan Nasrallah is scheduled to deliver a televised address on Thursday, is battling alongside regime troops in several parts of the country, including Qusayr in central Syria.