U.S. Secretary of State Rex Tillerson on Tuesday insisted that his relationship with Donald Trump was "good," though he admitted he disagreed with the president over the 2015 nuclear deal with Iran.

"He and I have differences of views on things like JCPOA and how we should use it," Tillerson told reporters in Washington, referring to the Joint Comprehensive Plan of Action, the deal's official name.

"But I think if we're not having those differences, I'm not sure I'm serving him," he continued.

On Russia, the top U.S. diplomat said neither he nor President Donald Trump is "very happy" about new congressional sanctions on Russia. He said the sanctions show that Americans are frustrated by Moscow, even though they want the U.S. to have a functional relationship with the nuclear-armed power. In a wide-ranging assessment of his first six months in office, Tillerson laments the sour state of relations with Russia and points to Russia's move to kick out many U.S. diplomats serving in Russia. Tillerson said he will meet with Russian Foreign Minister Sergey Lavrov during his trip to the Philippines starting Sunday.

Tillerson has reportedly become increasingly frustrated by Trump, who has often undermined his secretary of state with his tweets. Trump has preserved the 2015 deal between Tehran and major world powers, meant to constrain Iran's ability to make nuclear weapons for at least 15 years, but has been vocal in his criticism of it.

The pair has also clashed in other key areas such as environment. Former Exxon boss Tillerson was in favor of the US remaining in the Paris climate agreement but Trump decided to withdraw.

When the Qatar crisis erupted in June, Tillerson called for restraint on all sides, while Trump claimed partial credit for starting the feud between Qatar and its Arab neighbors. Nevertheless, he insisted they were on good terms.

Speaking to reporters at a nearly hour-long news conference at the State Department on Tuesday, President Donald Trump's top diplomat said he is comfortable in his job and in his relationship with the commander in chief.

Tillerson noted he has received Trump's confidence and said the president often calls at unusual times seeking input on various foreign policy matters. Tillerson said he wouldn't be doing his job if he didn't express his views, even when they conflict with those of the president.

"We have a good relationship," Tillerson said. "I talk to him just about every day, I see him several times a week, he calls me late at night, on the weekends when something comes into his head and he wants to talk. He may call me at any moment, at any time."

"It is a very open relationship and it is one in which I feel quite comfortable telling him my views," Tillerson said. "We have differences, but I think if we're not having those differences I'm not sure that I am serving him. I would tell you the relationship between myself and the president is good. That's how I view it, anyway."

Tillerson defended his plans to reorganize the State Department — a process that will likely see steep budget and staffing cuts — as necessary to keep the agency relevant in the 21st century. Those plans have been heavily criticized by former diplomats and academics who say he has "hollowed out" the building, destroyed the morale of career staffers and damaged the credibility of American diplomacy.