RIYADH: The United States and Saudi Arabia on Monday presented a united front to Iran and Syria. They warned Syrian President Bashar Assad that they will boost support to rebels fighting to oust him unless he steps down and put Iran's leadership on notice that time is running out for a diplomatic resolution to concerns about its nuclear program.

After a series of meetings in the Riyadh, U.S. Secretary of State John Kerry and Saudi Foreign Minister Prince Saud al-Faisal told reporters at a joint news conference that Assad must understand that recent scud missile attacks on regime foes in the city of Aleppo would not be tolerated by the international community and that he had lost all claim to be Syria's legitimate leader.

Saud, whose country along with other Gulf states is widely believed to be supplying weapons to the Syrian rebels, said Saudi Arabia could not ignore the brutality Assad is inflicting on his people, even after two years of escalating violence that has claimed 70,000 lives. He said that history had never seen a government use strategic missiles against its own people. "This cannot go on," he said. "He has lost all authority."

In his discussions with Kerry, Saud said he had "stressed the importance of enabling the Syrian people to exercise its legitimate right to defend itself against the regime's killing machine." Saud also decried the fact that the Assad continued to get weapons from "third parties," a veiled reference to Russia and Iran, which have backed the regime through the conflict.

"Saudi Arabia will do everything within its capacity, and we do believe that what is happening in Syria is a slaughter, a slaughter of innocents" he said. "We can't bring ourselves to remain quiet. Morally we have a duty." The Obama administration has resisted appeals from the Syrian opposition to provide it with weapons and ammunitions over fears that they could fall into the hands of Islamist extremists who have gained support among Assad opponents. But Kerry sidestepped a question about whether the arms reportedly being supplied to the rebels by Saudi Arabia and others were a concern. Instead, he criticized Iran, Hezbollah and Russia by name for giving weaponry to the Assad regime.

Kerry did announce last week that the U.S. would for the first time provide rebel fighters in the Free Syrian Army with non-lethal assistance - rations and medical assistance. European nations like Britain and France are expected to soon send the rebels defensive military equipment and Kerry has said the totality of the aid could be enough to change the situation on the ground.

"The United States will continue to work with our friends to empower the Syrian opposition to hopefully be able to bring about a peaceful resolution, but if not, to increase pressure on Assad," Kerry said. He added that Assad "is destroying his country - and his people in the process - to hold onto power that is not his anymore."

Kerry is in Saudi Arabia on the seventh leg of a marathon nine-nation dash through Europe and the Middle East on his first overseas trip as secretary of state. During his trip, members of the five permanent members of the U.N. Security Council and Germany met in Kazakhstan with Iranian officials in the latest bid to get Iran to prove that its nuclear program is peaceful and not a cover for atomic weapons development.

That meeting ended in an agreement for further expert-level discussions between the sides and both Saud and Kerry said it was critical for Iran to accept offers made by the so-called "P5+1" group quickly. Kerry reminded the Iranians that President Barack Obama has vowed not to allow Iran to get a nuclear weapon and that he has kept all options, including military options, on the table to prevent that from happening.

The window of opportunity for a diplomatic solution "cannot by definition remain open indefinitely," Kerry said. "There is time to resolve this issue providing the Iranians are prepared to engage seriously on the P5+1 proposal. But talks will not go on for the sake of talks and talks cannot become an instrument for delay that will make the situation more dangerous," he said.

Saud, whose country shares concerns with other Gulf Arab states about increasing Iranian aggressiveness in the region, agreed. "We hope that the negotiations will result in putting an end to this problem rather than containing it," he said, "taking into account that the clock is ticking and negotiations cannot go on forever."

In addition to Saud and the Saudi crown prince, Kerry met in Riyadh with the foreign ministers of Kuwait, Bahrain and Oman, all of whom are equally wary of Iranian intentions.

In a last-minute addition to his schedule, Kerry also saw Palestinian leader Mahmoud Abbas, who is visiting the Saudi capital. Kerry's working lunch with Abbas came two weeks before the secretary is to accompany President Barack Obama to Israel, the Palestinian territories and Jordan to explore ways of restarting the Israeli-Palestinian peace process.

Kerry said he would talk with Abbas about "all the obvious issues" and that Israeli Prime Minister Benjamin Netanyahu was aware of the meeting, which was being held on the same day that Vice President Joe Biden is to address the annual policy conference of the pro-Israel American Israel Public Affairs Committee in Washington.

From Saudi Arabia, Kerry heads into the homestretch of his lengthy first official trip abroad, traveling next to the United Arab Emirates and then Qatar before returning to Washington on Wednesday.