(c) 2013, Bloomberg News.

(c) 2013, Bloomberg News.

The following editorial appears on Bloomberg View:

Israel and Saudi Arabia strongly opposed the recent agreement to limit Iran's nuclear program, in part because they fear it will remake the Middle East to their detriment. It is up to Iran to prove them wrong, and that can start in Syria.

Iran's leaders claim that if they were treated as a regional power rather than a pariah, they could act constructively. If they can't demonstrate the truth of this claim, then the skeptics are probably right: In the midst of intensifying conflicts between Sunnis and Shiites, and between Iranian proxies and Israel, negotiated limits to Iran's nuclear ambitions will always be at risk of unraveling.

Iran can make its case by helping deliver an agreement that ends the bloodshed and suffering in Syria, where its Revolutionary Guard and Hezbollah proxies are fighting for President Bashar al-Assad and his minority Alawite regime. By using its influence with Assad to secure a settlement, Iran would, at the least, quell fears that it might use part of the cash from sanctions relief to boost its war efforts abroad.

For Iran to be tested in this way, though, it has to be invited to the table.

In announcing this week that the much delayed Syrian peace talks will start Jan. 22, United Nations Secretary-General Ban Ki-Moon rightly wanted to use the momentum of the Iranian nuclear agreement to break the impasse. However, Syria's fragmented opposition and the Assad regime haven't agreed on a basis for sitting down together. Nor has it been decided whether the international guest list can include Iran and Saudi Arabia, both essential outside players.

These talks should start small, much as the Iranian nuclear negotiations have done, putting aside at the start the hardest issues involving a political transition. The U.N. and Arab League special envoy Lakhdar Brahimi appealed this week for the two sides in Syria to carry out confidence measures, such as prisoner exchanges, even before January. That's a lot to hope for without outside pressure, given the escalating war on the ground.

One way to create incentives would be to organize a parallel Syria meeting Dec. 16 in Geneva, when the U.N. will be holding a major fundraising effort for crises around the globe, including Syria. Such a dress rehearsal for January could focus on securing on-the-ground access for humanitarian aid including by establishing local cease-fires and ending the siege tactics, used mainly by the Assad government, that have starved and terrorized whole communities. The scorched-earth measures used against civilians make it hard even for moderate rebels to take seriously the idea of negotiating with the regime.

Iran can show its good faith by convincing Assad that this has to stop, and this week's call made jointly with Turkey for a pre-January cease-fire was an encouraging start that should be followed up aggressively. Assad should also be pressured to order his forces to protect aid workers and medical staff, no matter who they are helping: 32 clearly identified volunteers working with the Syrian Arab Red Crescent have been killed during the conflict so far. Saudi Arabia, in turn, would need to work with Qatar and Turkey to persuade their rebel clients to do the same.

Most important for a lasting settlement, though, is that Iran and Assad should accept that the Geneva II talks will be based on the accord reached at the first round last year. That Syria agreement, hammered out primarily by the U.S. and Russia, stipulates the creation of a transitional authority with full executive powers, including over the armed forces a requirement that Assad and Iran have so far rejected. Whether Assad can be part of that transition is a subject for negotiation, but if Iran truly wants, as it has told Brahimi, to be part of the solution, it should start by accepting the only framework available to produce one.