U.S. to stay vigilant in monitoring Iran's actions

The U.S. will be " extraordinarily vigilant" in verifying and monitoring Iran's actions after the agreement on an interim nuclear deal takes effect, Secretary of State John Kerry said on Sunday.

Kerry made the remark after the P5+1 countries -- the U.S., Britain, France, China, Russia, and Germany -- agreed with Iran on Sunday to begin implementing a landmark nuclear deal on Jan. 20.

"We've taken a critical, significant step forward towards reaching a verifiable resolution that prevents Iran from obtaining a nuclear weapon," Kerry said in a statement issued by the U.S. State Department.

As of Jan. 20, for the first time in almost a decade, Iran's nuclear program will not be able to advance, and parts of it will be rolled back, Kerry said.

In response to Iran fulfilling its commitments, the P5+1 countries will also take steps to begin providing some limited and targeted relief, said the top U.S. envoy, adding that the 4.2- billion-dollar restricted Iranian assets will be released in regular installments throughout the next six months.

Kerry also called on the U.S. Congress not to impose additional sanctions that could threaten the entire negotiation process.

"Now is not the time for politics. Now is the time for statesmanship, for the good of our country, the region, and the world," he said.

Admitting that the next stage in talks with Iran will be very difficult, Kerry said, "We are clear-eyed about the even greater challenges we all face in negotiating a comprehensive agreement."