US Secretary of State John Kerry on Thursday warned against Russian missiles sales to Syria saying it would be "destabilising" to the region as he seeks to build diplomatic efforts to end the bloody conflict.

He also reiterated the US stand that Syrian President Bashar al-Assad has no role in any transitional government which emerges from growing efforts to bring the warring sides together to map out a political solution.

After three whirlwind days of talks in Moscow and Rome, Kerry is looking to end the stalemate in the 26-month conflict and bring the opposition and the regime to negotiations to end the fighting which has claimed 70,000 lives.

But the complex situation appeared to have been further muddied by reports that Russia is planning to go ahead with a $900 million deal to sell sophisticated surface-to-air missiles to Syria.

"We had previously stated that the missiles... are potentially destabilising with respect to the state of Israel. And so we've made it clear historically that this is a concern of the United States," Kerry said after talks with Italian Foreign Minister Emma Bonino in Rome.

"I think we have made it crystal clear that we would prefer that Russia is not supplying assistance," he told a press conference.

The Wall Street Journal reported Thursday that Israel has warned the US that Russia plans to sell sophisticated S-300 missile batteries to Syria.

The advanced ground-to-air weapons can take out aircraft or guided missiles, and could complicate any Western intervention to stop the bloodshed.

Meanwhile, Syria also warned it would "respond immediately" to any new Israeli attack on its territory, its deputy foreign minister told AFP, after two reported Israeli strikes on military targets last week.

"The instruction has been made to respond immediately to any new Israeli attack without (additional) instruction from any higher leadership, and our retaliation will be strong and will be painful against Israel," Faisal Muqdad said.

Kerry met earlier Thursday with Jordanian Foreign Minister Nasser Judeh to shore up US support for Amman, struggling under the weight of some 525,000 refugees who have fled across the border from Syria.

Jordan was working with the US 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," Kerry said.

He also officially unveiled $100 million (76 million euros) in additional US humanitarian aid for Syrian refugees, some $43 million of which will go to support UN programmes in Jordan.

Ten percent of the population in Jordan was now Syrian refugees, Judeh said. "It is expected to rise to about 20 to 25 percent given the current rates by the end of this year, and possibly to about 40 percent by the middle of 2014."

"No country can cope with the numbers as huge as the numbers I've just described," warned Judeh who later flew to Moscow for talks on Syria.

Since the war erupted, more than 1.5 million Syrians have fled to neighbouring nations, including Turkey, Jordan and Lebanon.

Up to four million more could be displaced within the country as they seek to flee the fierce fighting.

Kerry said there had been a "very positive response" to his plans for a new conference on Syria, aimed at finding a path towards a transitional government based on the six-point Geneva accord agreed last June.

Kerry and Judeh also discussed efforts to revive the Middle East peace process, with the US secretary of state expected to return to Israel for his fourth visit at the end of the month.

Jordan, one of only two countries to have signed a peace treaty with Israel, would play a key role going forward, Kerry said, adding it had also been instrumental in bringing together the Arab League to help kickstart the process.