U.S. President Donald Trump is "very likely" to state that Iran is adhering to its nuclear agreement although he continues to have reservations about it, a senior U.S. official said on Thursday.

Under U.S. law, the State Department must notify Congress every 90 days of Iran's compliance with the so-called Joint Comprehensive Plan of Action (JCPOA).

Trump has a congressionally mandated deadline of Monday to decide.

The landmark 2015 deal struck with Iran by the United States, France, Britain, Russia, China, Germany and the European Union put limits on Iran’s nuclear activities in exchange for removal of economic and financial sanctions.

If Trump does state Iran is in compliance, it would be his second time since taking office in January to do so despite his promise during the 2016 campaign to "rip up" what he called "the worst deal ever."

The official, who spoke on condition of anonymity, said Trump could always change his mind, Reuters reported.

While lifting nuclear-linked sanctions, the United States maintains sanctions related to Iran's ballistic missile program and human rights record.