The UN secretary general, Antonio Guterres, has warned that Iran may be defying a call to halt ballistic missile development even as it complies with the nuclear deal.

Guterres said in a report to the security council that the United Nations was investigating Iran’s possible transfer of ballistic missiles to Houthi Shia rebels in Yemen that may have been used in launches aimed at Saudi Arabia in July and November.

The report on implementation of a UN resolution that endorsed the July 2015 nuclear agreement was obtained on Wednesday by the Associated Press. The US mission to the United Nations said its ambassador, Nikki Haley, would hold a news conference on Thursday in Washington to highlight its findings, as well as Iran’s “destabilising activities in the Middle East region and elsewhere in the world”.

Q&A Why is Trump hostile to Iran? Show Hide The genesis of Trump’s particular antipathy to Iran is hard to pin down. Before entering office he had been sceptical of Iran’s regional rival, Saudi Arabia. But during the 2016 election campaign all his closest foreign policy advisors, such as Michael Flynn, shared a worldview that portrays Iran as an uniquely malign actor in the Middle East and beyond. After the election, Israel, Saudi Arabia and the United Arab Emirates were successful in capturing the ear of Trump and his son-in-law and top adviser Jared Kushner.

In the report, Guterres stressed that the nuclear deal remained “the best way” to ensure the exclusively peaceful nature of Iran’s nuclear programme.

He said President Donald Trump’s decision in October not to certify the agreement under US law created “considerable uncertainty” about its future. But he added: “I am reassured that the United States has expressed its commitment to stay in the joint comprehensive plan of action for now.”

Trump has left open the possibility of pulling out of the nuclear deal.

Guterres welcomed support for the treaty from its other parties, who include China, Russia, Britain, France and Germany, the European Union and others.

“I encourage the United States to maintain its commitments to the plan and to consider the broader implications for the region before taking any further steps,” he said. “Similarly, I encourage the Islamic Republic of Iran to carefully consider the concerns raised by other participants in the plan.”

Trump has called the agreement a bad deal, and the US has focused on its time limits and a provision in the security council resolution that calls on Iran not to undertake any activity related to ballistic missiles designed to be capable of delivering nuclear weapons.

Guterres said the UN was studying debris from missiles fired at Yanbu in Saudi Arabia on 22 July and at the capital of Riyadh on 4 November.

He said France, Germany, Britain and the United States had sent a letter saying the Simorgh space launch vehicle that Iran launched on 27 July, if configured as a ballistic missile, is “inherently capable of delivering nuclear weapons”.

But Guterres said Russia, an ally of Iran, sent a letter in August stating that the security council resolution contains only a “call” for Iran to forgo missile work – not a prohibition. He said Iran claims the launch vehicle was “part of a scientific and technological activity related to the use of space technology” that it is determined to pursue.

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The security council discussed the launch in September, and “there was no consensus among council members” on how it related to the 2015 resolution, Guterres said.

He said Israel had protested that Iran’s test of a Qiam ballistic missile in November 2016 “used a Star of David as the intended target” and cited other ballistic missiles it reportedly launched at targets in Syria in June. France, Germany, Britain and the US also raised these tests, as well as the test of a medium-range missile on 4 July.

The secretary general said Iran had called Israel’s claim of a marked target “a sheer falsehood”. Iran also said its “military capabilities, including ballistic missiles, have not been designed to be capable of delivering nuclear weapons and thus are outside the purview of the security council resolution”, Guterres said.