However, these significant achievements do not necessarily suggest that it is definitely possible to make the final agreement before the end of June. The core challenge will be the removal of U.S. unilateral sanctions on Iran. The U.S.-imposed sanctions on Iran's oil exports and finance sector, which cuts off Iran's U.S. dollar payment channel, have most seriously undermined Iran's national economy, so these sanctions are the ones that Iran most desperately wants to have removed.

Unfortunately, these two sanctions are the least likely ones to be removed by the U.S. Although the U.S. can openly argue that the removal of those two sanctions will depend on Iran's implementation of the agreement, the White House cannot actually remove the sanctions at all without the approval of Congress, which is dominated by anti-Iran politicians. The White House can only suspend U.S.-imposed sanctions for up to 24 months according to a bill passed in 2012.

John Kerry reportedly discussed this issue with Zarif while in Lausanne. Kerry promised that the White House will negotiate with Republicans to remove sanctions once the deal is reached, but he could not guarantee that he will actually be able to deliver on the promised lifting of sanctions.

Therein lies the problem. As long as Iran knows that the U.S. likely cannot remove its unilateral sanctions, how can Iran trust that the U.S. will not find some pretext to claim that Iran is not fulfilling its obligations in order to escape its own duties? A letter to Iran's supreme leader signed by 47 congressmen asserting that Obama's deal with Iran will not be honored by future U.S. presidents certainly added to Iran's legitimate suspicions.

Assuring Iran that the U.S. can deliver on its commitment to removing the above-mentioned sanctions will be a critical challenge for U.S. negotiating teams. Will John Kerry be able to honor his promises? It is a big question.

As the JCPOA parameters of April 3, 2015 and the JPOA of Nov. 24, 2014 both stipulate, nothing is agreed until everything is agreed. That means that if the parties fail to address this issue before June 30, 2015, all the efforts invested previously will be of no avail. The real challenge will be on the U.S. side.

On one hand, the week-long diplomacy session between Kerry and Zarif signifies hope for a positive final outcome. On the other hand, it also suggests that both sides cannot easily make concessions. For John Kerry, the question is whether he can commit to something he cannot actually deliver, while for Zarif, the question is whether he and his boss, Hassan Rouhani, can accept an obviously flawed agreement.

The writer is a columnist with China.org.cn. For more information please visit:

http://china.org.cn/opinion/jinliangxiang.htm

Opinion articles reflect the views of their authors, not necessarily those of China.org.cn