Iran is failing to fulfill the "spirit" of its nuclear deal with world powers, President Donald Trump has declared, setting an ominous tone for his forthcoming decision about whether to pull the U.S. out of the landmark agreement.

On Iran, Trump and his top officials have been walking a narrow line as they seek to show an aggressive stance. U.S. Ambassador to the United Nations Nikki Haley accused Iran and its ally Hezbollah on Thursday of conspiring to destabilize the Middle East — a charge "categorically" rejected by Iran's U.N. envoy as a misleading propaganda campaign perpetrated by Israel and others in the region. Haley told the 15-member council that its monthly meetings on the Middle East - traditionally focused on Israel and the Palestinians, Syria and Lebanon - regularly turned into "Israel-bashing sessions."

While disparaging the nuclear deal and accusing Iran of fomenting violence and terrorism throughout the Middle East, Trump has avoided committing to abandoning the nuclear agreement. U.S. Secretary of State Rex Tillerson on Wednesday accused Iran of "alarming ongoing provocations" to destabilize countries in the Middle East as the Trump administration launched a review of its policy toward Tehran, including a 2015 nuclear deal.

U.S. Defense Secretary Jim Mattis visited Israel on Friday for talks expected to focus on Iran, Syria and the two countries' close strategic relations despite recent tensions with Barack Obama's administration.

U.S.-Iran relations have become more hostile than ever as the White House takes an aggressive posture toward Tehran over test-firing a ballistic missile that could raise tensions in the already chaotic region.

Iran, which has been accused of exposing sectarian fault lines in the region, especially in Yemen, Syria and Iraq, tried to soften its rhetoric as Rouhani said there should be greater unity between Shiites and Sunnis and that they had coexisted side by side peacefully for hundreds of years. Iran continues to send advanced weapons and military advisers to its Shiite allies in Syria, Iraq and Yemen while stepping up support civil wars whose outcome could sway the balance of power in the Middle East.

During the Obama era, Iran enjoyed the opportunity to fill the vacuum in the Middle East after the White House abandoned its traditional allies, like Turkey and Saudi Arabia. The Trump administration has not been tolerant of Iran, contrary to the former administration. Referring to Iran as "the number one terrorist state," the U.S. president said the Middle Eastern country supplies money and weapons to terrorist groups.